I would suggest borrowing and trying a Giro Prolight in size S. The strap construction makes it the most form-fitting of any Giro I have tried. In fact it felt tight at first but now I don't notice it.

Before Giro moved to their current S/M/L sizing, they offered helmets in XXS/XS, which were a bit tighter than the current size S. Another idea is to wear a do-wrap/skull cap to fill out your helmet a bit.