The truth is that there are some people out there who are just naturally blessed with more muscular legs and they don't have to train them as hard in comparison to the average person.
There are guys out there with impressive quads and hamstrings who don't even consistently train with weights at all. Then on the other end of the spectrum are those with naturally thin legs where adding significant size is going to be more challenging. So there's for sure a genetic component involved. However, if you're seeing consistent gains in your other muscle groups, but your legs are the ones that are lagging behind, the real reason for this is usually pretty straightforward. The reason, assuming you already train with sufficient volume and you're using proper form with a full range of motion on your lifts, the reason is that compound leg exercises are much more demanding on your body as a whole because the quads, hamstrings, and glutes are very large muscle groups and they're capable of moving a lot more weight in comparison to other muscle groups. And so most lifters without even realizing it, they don't end up training their legs within the same proximity to muscular failure as they do with their upper body muscles, which are generally just a lot easier to train.
How many Reps?
So maybe you're averaging say one to two reps short of failure on most of your upper body lifts, but then when it comes to leg training, you're actually leaving four or five or even more reps in the tank on your sets, but you're perceiving it as being closer to muscular failure because your body as a whole is under a lot more stress during the set just from having to move that larger amount of weight using your body's largest muscle groups. And so the total perceived stress, say four rep short of failure if you're doing a barbell squat, that feels like the same level of difficulty as going maybe two reps short of failure on a weighted pull up, or going all the way to failure on an isolation exercise like maybe a bicep curl or a lateral raise.
So what often ends up happening is that you end up cutting the set off at that point, even though your actual leg muscles, the muscles that you're trying to target for hypertrophy, they still have enough strength to continue further but that total body stress is what stops you from continuing, so in very simple terms, you're just training your legs as hard as you're training everything else, even though it might seem like you are.
Be as big and as strong
I'll even call myself out on this. Back in my bodybuilding focused days where my goal was to be as big and as strong as I possibly could, I used to train my legs really hard. I'd go into the gym on leg day. I was nervous, super focused. I'd psych myself up, get into the squat rack and how difficult it felt was irrelevant to me. Every rep was just a maximum effort and I knew how close I was to failure and what the goal of the set was, and it was just a matter of getting the reps done, no matter how challenging it felt. Back then I had pretty well developed legs. They didn't lag behind my upper body, and it was because I treated my leg training in exactly the same way as I treated any other muscle group, which is something that a lot of people just don't do. It wasn't about perceived total body discomfort like, "Damn, this bar on my back feels so heavy. My shoulders are burning. I'm breathing heavily. My back feels fatigued, so I guess I should just stop now."
It was just about how close my actual leg muscles were to muscular failure, and the set continued until I hit the level of effort that I was aiming for regardless of anything else, whether it was two reps short of failure, one rep short of failure, or all the way to complete muscular failure. Push it...
Bodybuilding hasn't really been my goal
But since bodybuilding hasn't really been my goal for probably the past eight years or so, and I don't take it quite as seriously anymore because I've been doing it for such a long time and also because my desired physique is more of a middle ground look anyway, because of that I just stopped training my legs as hard as I used to. You know, I started getting a lot more casual with my leg workouts, still going within a few reps of failure, still training hard, but not truly buckling down and really hitting them with the same level of effort like I used to, but still training my upper body fairly hard again because it's generally just an easier set of muscles to hit and it doesn't require as much mental toughness. As a result of that, my legs did lose a noticeable amount of size and they do lag somewhat behind my upper body now. Not horribly, but the difference is pretty obvious if I just look at my physique and if I look objectively at my training, the reason is pretty obvious for that as well. That's something that I'm working on correcting right now because I just had to look at what.
I was doing and admit like, "Hey man, you know, you're just not training as hard as you could be in this particular area."
Everyone has different goals and different preferences
Now, of course everyone has different goals and different preferences and they treat their training with different levels of seriousness, so I'm not saying it's a given that you should want to build the biggest legs possible or that there's some sort of ideal physique proportion that you must aim for. But what I am saying is that if your goal is to build bigger legs and they aren't really responding that well even though your other muscle groups are, then I would wager that the primary reason for that is that probably without realizing it, you're just leaving more reps in the tank on your lower body exercises than you are on your upper body lifts because leg exercises are generally a lot more difficult and what you're doing is cutting the set off based on general total body discomfort as opposed to where your quads and hamstrings actually are in relation to true muscular failure.
If your goal is to maximize hypertrophy
I would say that if your goal is to maximize hypertrophy, then on the bulk of your sets you should be aiming for about one to two reps short of failure, and I would say three reps at the most. Meaning that if you were lifting with let's say your ten-rep max, you'd want to be doing anywhere from seven to nine reps or if you're lifting with your eight-rep max, you'd want to be doing anywhere from five to seven reps. So next time you're in the squat rack and you've got that loaded bar on your back and you're uncomfortable and your mind is telling you to stop, or you're on the leg press and you're moving that heavy load that most people usually are capable of moving on the leg press, or even something a during something like a Romanian deadlift. Try to be honest with yourself and ask if there was a gun to my head right now, how many more reps could I truly squeeze out in proper form if I exerted myself with maximum effort, regardless of how uncomfortable I feel in this moment or how difficult it is? If the answer is any more than about one to two reps, again, three at the most, then you're just not training your legs hard enough and you need to mentally buckle down and start pushing yourself closer to the point of muscular failure if you really want to develop your legs optimally.
Video of Sean Nalewanyj