Where did you get the employee numbers from? Are they even current?
Anyway considering that we don't know how many games are under concurrent development at Turn 10 nor whether that employee headcount included contracters needed to finish a game or regular FTE numbers we cannot make any real comparison between them. I know that Turn 10 did mention they were interested in making other games aside from racing games however.
Actually not that current. Turn10 had 200-250 ppl on Forza 2 and 300-350 on Forza 3. Polyphony has 110 developers, but in peaks they had 140 ppl working on GT 5 and they started with 50 team members after GT 4.
http://www.gamasutra.com/view/feature/4144/racing_evolution_forza_3_and_the_.php