If you haven't already, you may want to search this site for dysmorphic. There are quite a few fellows who dealt with that issue and perhaps there was something in their stories which will ring true with you.
Misery loves company
Your forehead probably looks bigger to you because you are receding.
I just realized when i look at myself in the mirror or a photo of my self I mainly just look at my forehead/hairline maybe its the fact I'm obsessed with it