Some of us are naturally skinny; some of us are more bulky. Bone thickness and structure will also differ from person to person (which needs to be taken into account when designing a plan for building muscle). Traits like our muscle belly length and metabolic rate are also important.
When it comes to improving your physique, the better you understand your particular combination of traits, the greater your chances are of building the physique you're after. A cookie-cutter approach to building muscle just doesn't work.