A Mystery, Indeed!
Throughout the history of the incredible hulk, one would think that there'd be a wardrobe mishap somewhere. Especially considering the fact his shirt, shoes, or anything else other than his pants are always torn to shreds during the transition.
However, in the latest hulk film I noticed that in human form Bruce wore a lot of jogging pants/sweatpants. Maybe its somewhat possible that these stretched with him during his change?
It still doesn't explain the various incarnations of the hulk before Mr. Norton, though.