Why is it that some people appear to make rapid gains in muscular size and strength when they lift weights, while others seem to make no gains at all? A lot of variables enter into an equation that seeks to answer this question. One example is the natural level of testosterone a person has. Testosterone is without question strongly related to muscular size. This was proven once and for all in a 1996 study published in the New England Journal of Medicine. The study showed that men who injected 600 milligrams a week of testosterone gained significant amounts of muscle. Exercise added to the effect, but even if the men didn't train, they still gained more muscle mass compared to the group that trained but didn't inject testosterone. Up until the publication of that study, the use of anabolic steroid drugs, which are synthetic, altered forms of testosterone, was controversial. In spite of the obvious physical effects produced by steroid use in athletes and bodybuilders, stubborn holdouts in the medical profession still clung to the long-discarded notion that steroids didn't really help build muscle, but rather just caused "water retention." But with the publication of the New England Journal study, all doubts about the anabolic effects of testosterone were quelled. These days the only debate about testosterone and anabolic steroids relates to the long-term health effects of the drugs. You can read the original 1996 study proving that testosterone builds muscle here: http://www.nejm.org/doi/full/10.1056/NEJM199607043350101#t=article.
When you begin training with higher natural testosterone levels, muscular gains come quick and easy. Those with higher testosterone levels who respond rapidly to training are often referred to as "easy gainers." This doesn't mean that you won't gain any muscle if you start with comparatively lower testosterone levels, but rather the initial muscle gains may develop a bit slower.
Another factor that determines response to resistance training exercise is the genetically determined ratios of your muscle fibers. If you are born with more muscle fibers in any particular muscle, that muscle will be more prone to gaining muscle mass and strength. Although the proportion of muscle fibers would need to be determined by muscle biopsies, which involves the removal of small bits of muscle tissue that are then examined under a microscope, you can get a rough idea by looking at the length of your muscle bellies. The longer the muscle between its points of origin and insertion, the greater the number of muscle fibers. A good example of this is the calf muscles. Longer calves have a far greater potential to grow and gain muscle mass compared to "high" calves. This type of genetic anatomical variation cannot be changed.
What is under greater control is exercise style. I train . . .