If a game takes 5 years to make and is only 20% done at the end of that, sorry, that means they are too small even with 140 people. Only about 30 or so, by my estimates of the end credits, are car modellers. I highly doubt more people are going to harm the quality of the product (see: Ass Creed 2, a better game all round than AC).
I'll take a stab at an analogy - a penis that is 7 inches is not small, it is quite big actually. But for a woman with a massive 9 inch deep cunt with lips flapping about like a Sarlacc - it isn't going to be big enough.
Polyphony Digital is that penis.