I've been going to gym for 4 weeks now and I've noticed that there are a lot of people there who lift way more than me yet they are smaller than me. I've talked briefly with some of them and their goal is definitely size (and they actually do 6-10 reps average). Are they doing something wrong or is it just difference in genetics?
You've only been going to the gym for 4 weeks. It takes time to ramp up in weight, and ramping up in weight is not as important as having proper form.
As long as the smaller people are using proper form, it's fine that they are using heavier weights. However, it's often the case that people use improper form and high weights, which can result in injury.
I would worry about maintaining your form with your current weights, and once you feel comfortable enough with those weights, make slight increases. Also, try not to watch what other people do in the gym, which is often for show.
Actually you should be worried more about your form when you perform the exercises. If your form is not good enough you can lift more weight but all to no avail. You will just trick yourself you are better and making progress.
As for the weight the rules is more weight, fewer reps (80% percent of the max weight you can lift 3 series of 6-8 reps) for a set. If you can do 10 reps increase the weight.
Some people have been doing this for a lot more time then you and some people have different genes. It is natural they that they lift more weight then you now. You shouldn't be worried about that. If they are smaller in size, this doesn't mean they have less muscle tissue.
There are multiple possibilities here. They are: -The people have short arms so it is easier for them to bench more -They are short but have built up muscles -Your diet is poor and sleeping habits are poor and theirs is good -They have been training much longer than you -They are doing different exercises than you which has built muscle faster -Their technique/form is poor Of course... Size doesn't always determine who is stronger. For example, a 5'8, 140 pound person might be able to lift more than a 6'4, 250 pound person if they train harder and even if they train equally hard, genetics always play a role to some degree. Height and weight can but does not have to determine physical strength.