|MadSci Network: Anatomy|
High Intensity Interval Training (HIIT) by itself does not necessarily increase muscle mass at all. Lots of people use the techniques in a different way to lose weight rather than gain it. However, to the question as to why lifting a heavier weight fewer times builds muscle... The quick and easy answer is that lifting a lighter weight is easier! There's a difference between tiring your muscles out and stressing them. In order to build strength and bulk, one must stress the muscle by operating it beyond what it's used to. This causes it to adapt by building more myofibril tissue, more connective tissue, and so on, in order to make it so that the next time you try to stress the muscle, the muscle will be able to respond better. It's simple adaptation to environment, essentially.
Try the links in the MadSci Library for more information on Anatomy.