because she eats substantially more calories than she burns
since the body has adapted from primitive times of scarcity when presented with excess sugars instead of disposing of them it will instead convert them into a longer lasting form (sucrose) then into a long term storage (glucose). ironically each time the body converts these sugars they also become progressively harder to break down so the body will often prefer to use reserves of sugars and sucrose first before breaking down its stores of glucose. This means if you live an all american diet of all sugar all the time your body will never really be presented with an oppurtunity to break down stored fats.
its as Dear Leader stated its simple maffs, if you take in more calories then you burn your body will store them