。
She isn't fat exactly, but she's a big-boned person.
Literal
She [topic-は] is-fat [quotative-とは] cannot-say [even-までも], large-build [genitive-な] person is.
~とはいえないまでも is a layered concessive: ~とは (quotative + topic) + いえない (negative potential of 言う) + までも (concessive 'even if not'). The whole means 'while (one) cannot quite say X, still Y / not exactly X, but Y.' 大柄 ('large build, large-boned') here is the おおがら reading (not the unrelated おおへい 'arrogant'); paired antonym is 小柄 ('small build').