I's know not the scientific logic behind this but the reasoning goes: increasing the weight means the more resistance put on the muscle. The more resistence equals the more muscle fibers torn and the more that rebuild, thus increasing the muscle mass.
Plus the challange to push one's self further.
And you must always remember to rest and stretch between sets, to decrease the chance of serious injury and increase post workout recovery time.
But that is what I's merely gleamed from my's experience, whilst others may have different views and experiences on this matter.
If so, I's would be intersted to read them.
Peace
The Pimp NeonBlack

First, spend a little time reading some bodybuilding sites.  That'll give you an understanding of the commonly-held principles and basic physiology.  Once you have a basic grasp of the physiology (e.g. you know roughly what a sarcomere is), spend some time searching pubmed.com and reading abstracts there.