@simon_brooke @wonderofscience @Physicsj I'm no expert, but it is not just mass. Closeness to the sun (close planets rotate slower) and possibly the existence of moons play a role. I also looked it up and apparently, no one knows exactly why Venus rotates so slowly (also compared to Mercury) and in the 'wrong' direction (the only one besides Uranus to rotate to the left instead of the right in the picture above).
Thank you, I learnt something new today.