Not everyone's body changes at the same time or even at the same pace. It can be tough if all of your friends have already matured physically and are taller and more muscular. Most guys eventually catch up in terms of growth, although some will always be taller or more muscular than others ?it's in their genes.
It's natural to observe friends and classmates and notice the different ways they're growing and developing. Guys often compare themselves with other guys in certain settings, and one of the most common is the locker room. Whether at a local gym or getting ready for a game at school, time in the locker room can be daunting for any guy.
Try to keep in mind in these situations that you aren't alone if you feel you don't 'measure up.' Many guys feel exactly the same way about their own bodies ?even those whose physiques you envy. Just knowing that almost everyone else will go through the same thing can make all the difference.
You could try talking to a trusted male adult ?maybe a coach, a doctor, a teacher, or your dad. Chances are they went through similar experiences and had some of the same feelings and apprehensions when their bodies were changing.