Each airline decides what their baggage policy is. Disney has absolutely nothing to do with that.
I see so many people traveling with huge amounts of checked baggage. Not sure why. You can do laundry at WDW. Every resort has laundry facilities..usually close to a pool area.
Unless you're going for a really long time, over 10 days, you shouldn't need two bags worth of clothing. I've actually been gone for two weeks, with 2 nights in Cocoa Beach, 4 nights on the ship, 5 nights in WDW and then 3 nights in Tampa...only took 4 bags for 3 people. I did pack a foldable duffle in one bag just in case we needed it.
People tend to pack bags using the 'what if' theory. You usually don't need all that 'stuff'.