The simple fact that Alice literally slips on her own ice while she was monologuing to Iska when they first met (in the anime, the small outcropping she's standing on breaks under her feet). This kickstarts their relationship as she fell into Iska's arms bridal style.
Mismis is basically a walking joke in many situations, from losing constantly at a casino, to being unable to run without the weight of her boobs throwing her off balance, to outsiders not believing she's actually 22 years old, to her various silly reactions upon being reminded of her woeful situations (i.e. becoming an Astral Mage), and so forth.
When Alice chains herself to Iska in order to appease Rin, she claims she'll be with him at all times. Iska then points out that would also mean he'd have to join her when she has to go to the bathroom. Her reaction is hilarious.
When Alice accidentally walks in on Iska after getting out of the shower, her reaction is understandably one of mortified shock. A few minutes later, she claims that in order for things to be equal between them that he should strip for her. Both Iska and Rin think she's lost her marbles.
In the manga, there's one panel where Elletear is having a conversation with Lord Mask, and several of his maid servants are in absolute awe at the shadow being cast by Elletear's enormous breasts. Their expression is what makes it funny.
