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.
Golems cannot ride minecarts, this is an intended change from several versions back.
Please do not mark unreleased versions (anything starting with "Future Version") as affected. You don't have access to them yet.