When I joined I was 5'9, 125lbs. I was by far the smallest guy on my basic and I won't lie to you, that's not a good thing. For the first few weeks instructors were all over me saying I was too small to be in the army and should VR etc etc. Coursemates didn't make fun of me, but you could tell there was always that "lack of respect," if you may, that comes with the territory of being a small guy.
After a few weeks, however, instructors began using me as the example of "Pvt. ******** is the smallest guy on this platoon and he is having no problems with this!"
Pretty much the same story for when I did armoured school and when I got to regiment. I've since bulked up to about 150lbs and look a little more filled out. Don't concern yourself with your size compared to others. We all have a job to do and if your section commander, or whoever is in charge of you is an effective leader, he will recognize the strengths and weaknesses of everyone in the section and TRY to assign taskings appropriately.
Being smaller I'm quite good at endurance things, as well as moving quietly through the woods on a dismounted recce compared to the bigger guys. That being said, give me a C6, ruck, Carl G, and ammo.....although I CAN carry it, I can't go as fast or as far with it as the bigger guys.
So like I said, we all have roles, don't concern yourself with trying to compare yourself to someone bigger who fits a different role. It's almost like apples and oranges. Worry about passing your BMQ, BIQ and being an effective member or your regiment when you get there by giving 100% and not complaining about things.