They carry their weight higher than a 450, so they will feel more top heavy when compared to a 450. I race XC and have no issues with the extra weight, and I only weigh 130 lbs. Having a reverse gear was a must have for me.
The 700 motor weighs more than a 450 and is taller. The radiator is mounted higher too. This is why they feel more top heavy than a 450. A 700 has the potential to make more power and it comes in at a lower rpm than a 450. The 700 builds rpms slower than a 450, because the piston is very large, it has more reciprocating weight in the motor and a longer stroke.