Why is my girlfriend doing this?

ok so my GF and i are both 15 and have been going out for 3 months now. so when ever we hug and kiss she likes to grab on to and hold my fat. she also likes to sit on my lap shes says im soft and comfy and she said to me "Santa Claus has got nothing on you" i thought that was funny. im 5'8 and 255 pounds so i know i am fat, and shes not fat at all she is a small girl shes 5'0 and im going to guess around 100 pounds. her past BF have been averaged sized im her first big BF. i really dont care that shes does it im just curious.