I have that same problem too.
I recently got myself 1520 and during charging and prologend use (game, ie.,etc..) my top left corner get heated up and warm. Is that normal?
My gf 630dual is warm after intese use too..but my 735 is not. Maybe just because it has ticker plastic case, i don't know.
Sorry if i hijacked but we have same problem.
Lumia 1520.1 8.1 denim