It'll usually boil down to diet, and experience. Handling weights, besides obvious physiological changes, takes practice. The more one practices, the more familiar they and their body gets to handling what's going on. I was one of those guys that was not as shredded as my training partner, a lot older, but yet I can always haul more weight, just not as long. Size and appendage length can play a big role for strength in certain exercises. In particular, the bench press. Guys with a shorter distance to raise the weight can handle much more off the git go. Shorter arms versus a tall dude. That doesn't always hold water, but if you took two guys with same training experience and diets, that theory can prevail. Also, don't dismiss the mind's eye either. If someone is very focused and in that 'heightened state of awareness' when training, glorious things can happen. Tiger Woods speaks of this state of being when he's really grooving. He won't remember much of anything, external noises and distractions disappear, things happen seemingly effortlessly. This however is from deep commitment and practice and can be applied in the gym, no doubt. So if a small person is ready, able, and willing, anything is possible.