Im sorry if im wrong but im sure thats not mylar, For your heating problem as I said sorry if im wrong its probly because you've got your setup with tin/aluminum foil. If it is mylar then its either old stuff or its shitty mylar, Also with your reflecting you also wanna make sure if your using mylar to not make folds and crumples in the mylar because when the light reflects off of it, the light creates hot spots because theres bends in the mylar :P

Aswel to your problem with it small, What kind of nutes are you using? Also maybe try using some stronger lights.