aww heather i'm sure she just has no idea that the stuff she's buying is too small. and you're right if she's that tiny an XL will look big to her.
or it could be that she loves you, and because of that, is just blind to how big you are. when i've told friends my size or weight they always seem absolutely shocked. or if i show an old *really* fat pic of me, no one can ever seem to remember me looking that way. i think it's just what happens when you actually see someone for what they are inside instead ... you stop noticing the flaws altogether when you see the good in a person.
[but damn it would be nice to shop in a normal store again!]