As I understand it is already fullscreen. It's centralized, but I understand what you mean. Probably the app would need to be rewritten and maybe game would look a little 'pixaleted' as NeoGeo games are about 320 x 224 px . You can use TV Out for better viewing.

PS.: It was my 100th post and I have 10 thanks, that way I'll have 100 thanks when I have 1000 post, long way to go...