Why is my forehead bigger when I put on glasses?

Your forehead may appear bigger but it probably is an illusion when you wear glasses.  Or... your may have better eye sight to appreciate a bigger forehead?  Or ... as you noted it is just a perception when you wear small or tight glasses versus larger glasses.

This is an issue of perception. You see a larger forehead with glasses so maybe you are too sensitive about the glasses. Speak with members of your family about this perception or people you can trust.