A unicorn's horn was a large part of what made them beautiful. The longer and pointier it was, the more attractive it was considered. Rarity was fortunate and blessed enough to be considered 'well endowed', despite her pedigree. She'd always been a little jealous of Twilight after her Alicorn transformation; and made every effort not to blatantly ask to 'compare size' with her like some immature schoolfilly who wanted to know for sure who has the best. Rarity was sure it was herself, of course. 