Why do moms buy clothes that are too big?

My mom follows me around the store, I pick up a pair of size 12 1/2 leggings. I come out of the dressing room, and I look fine. My mom leaves and comes back and says, "How about trying on this?" Its the same leggings, just in a size 16. I ask her why she does this and she says that all the clothes I am wearing are just too tight. I swear they are not, and she always gets me what I want, just not the size I want. How do I confrount her and actually get an answer? Let alone, get clothes that fit me!