She's only put on a pound or two, really!
Rarity, or Princess Rarity if you would please thank you very much, is an alicorn ascended to Princess status purely in an attempt to give Applejack something to really complain about. After a brief Jabba-like stint of overindulging on the cakes at her disposal, ended when Spike delivered a regretful but painfully accurate assessment of her actions, she's slimmed back down to acceptable size and vies frequently for the heart of her favorite dragon.
She has refined and modified Spike's signature weapon 'Charity' with enchantments and crystals of her own making, making it more precise, deadly and able for her telepathically communicate with him at will.
Just a little pudge, that's all!
She wanted to be a princess, but somehow this part got left out of the brocures.