Later on the novel, the MC discover that Britney's body took an habit to overeat and crave for sweets when stressed (to be more precise due to her growing environnement, the former Britney became a clinical textbook exemple of a child afflicted with stress related bulimia), so even with her best efforts she rebounded quite a few times when subjected to heavy stress, and had to fight hard for each kilo (or pound if you have issues with exactly divisible mesure units 

 ), dropping again her already rock bottom self esteem