Main Question or Discussion Point
Quick question, do bigger planet spin faster?
Planet Speed at equator (km/h)
You would expect small planets to spin faster - from conservation of angular momentum
But larger planets were formed from larger clouds of stuff.
As stuff contracted, then because of conservation of angular momentum it speeded up, the bigger/faster the original cloud the faster it ended up. Of course if you took the existing planets and made each of them smaller - they would speed up even more.
Then there are effects that have happened since. Mercury's rotation is slowed by tidal friction with the sun so has a very slow speed (long day), the Earth's is slowed a little by friction with the moon.
Uranus probably got hit by something in the past - which is why it has a weird axis tilt.
Mars might also have been affected by whatever caused the asteroid belt.