Japanese don't air laundry of their employers, for one thing. For another, every article I've ever read states that Gran Turismo games make huge profits for SONY, not Polyphony. Thirdly, yes, the size of PD is in the hands of Kaz, but the source of his budget seems to be SONY.
However, they have grown over the years. Back in GT4 days, I think the company size was roughly 110 employees, and at the end of GT5's development, Kaz mentioned 130 in an interview, and people have found Polyphony employment ads here and there, so current employee size isn't known. Lastly, you'll have to point out a number of low budget developers struggling with poor sales having more people, because I'm just not aware of many.