She's really just a poor dresser. Her outfit choices are for girls who are a size 0 while she's really a 4-6 so they make her look bigger than she actually is. Now I love a good Xtina fat joke but I can't sit here and say that she's actually fat.
I hope for the new era she wears clothes more appropriate for her figure and stops wearing leggings altogether. They do nothing for her body and make her look sloppy. She needs a stylist who understands proportions because clearly this guy does not.