It's just a fit issue. In our car at least, 8 inches is the maximum motor diameter that will fit and still leave room for the right side axle to pass from the diff, behind the motor to the right wheel.
You can get an idea of how close things are in the ForkenSwift in this video @ about 49 seconds - look at the right axle turning behind the motor.
In a RWD car, you'll have tons more lattitude on motor size. And I've even seen some mods done to the firewall / transmission tunnel to permit a big motor in a RWD vehicle. Much easier to do that if you're oversize than sort out axles in a FWD situation.
There are probably some larger FWD cars that would have plenty of room. But the key is to be aware of the distance from the center of the input shaft to the axle that passes behind the motor.