Iron Golems can't ride Minecarts

Most of the time, when a mob is pushed into the same space as an empty minecart, the mob starts riding the cart (and will thus be moved by rails, etc.) However, if an iron golem is pushed into this space, it will simply move the minecart away.

This is not a result of the Iron Golem's size; spiders and ghasts can ride minecarts without any problem.