I'm male 90kg 172cm. I've been going to gym for about 3 weeks. It seems like my legs are weak compared to upper body. For example I could hardly do 4x15 squats with 40/45kg during 1/2 week, while I bench press about 70/75kg. (from my observations all people in the gym squat more then bench press) A friend of mine says I should increase the weight for squats and reduce reps for time being. What's the logic behind this advice? Is it better to slowly increase the weight keeping current sets and reps?
Reps in the range of 12+ tend to be geared more towards muscle endurance than hypertrophy (more muscle mass) or strength.
A vast number of training programs with a trainee's 1-rep max (1rm) in mind, this is the most that the trainee can lift one time before failure.
As shown in the image, working in a lower rep range will provide your body with a different stimulus that will promote overall strength gains. That doesn't mean that you have to work in the lower rep range, but progress will be slower if you stick to 15 reps for all of your lifts. This page provides various links that will allow you to explore the effects of the different rep ranges. You can also use the ExRx website to help estimate your 1rm to better plan your routines / overall program.
In the end, what you decide to do in terms of increasing the weight for your squats while decreasing the number of reps, is ultimately up to what your goals are. If you're looking for maximal strength then sticking to sets of 3-5 reps might be best, while going for size will benefit from having sets with 8-12 reps.
15 is a very high number of reps compared to what most gym-goers do, it's also in fact, as Alex L showed, a very endurance focused rep range.
There are three main factors which affect how many reps you can do:
I. neurological adaptation This is your nervous systems ability to make your muscles do their work at the highest possible strength. When you work out at 1-5 reps, your nerves will perform better.
II. Muscle fibre type 1 These are your slow twitch fibers, they are the ones doing the work when you need endurance, when you do 50 body weight squats etc. When you work out at 12+ reps, you will get increases in these fibers mostly.
III. Muscle fibre type 2 These are your fast twitch fibers, they are used when you need to lift heavy, but they also tire out a lot faster than the type 1 fibres. When you work out at 1-5 reps, you will get increases in these fibers mostly.