Actually, at last count, they were about even (around 15 each). This is particularly better for Sony [currently], given their 1st Party studios put out more hardware pushing content.
Of course, I expect things to even back out some at the beginning of next gen, but I just wanted to clear up that little misconception. :P
Not to mention some SIE studios have multiple teams within them (Japan Studio, Insomniac, Guerilla Games) and Sony have been expanding their in house teams for a few years now.