What kind of food have you been eating to make you gain so much weight?
この場合は現在完了進行形を使って表現すると良いでしょう。
ーWhat have you been eating to make you so big?
「何を食べてそんなに太ったの？」
ーWhat kind of food have you been eating to make you gain so much weight?
「どんな食べ物を食べてそんなに太ったの？」
to gain weight で「体重が増える・太る」
ご参考まで！
カナダ 
