A proverb joking that even when you're full, there's always room for sweets/dessert ('there's always room for dessert').
もうお腹いっぱいと言ったのに、ケーキを見ると『甘い物は別腹』と言って食べた。
I said I was full, but when I saw the cake I said 'there's always room for dessert' and ate it.
彼女はいつも甘い物は別腹だから、デザートまでしっかり食べる。
She always has room for dessert, so she makes sure to eat it too.
みんなで分けようと言ったが、結局『甘い物は別腹』で一人で食べきってしまった。
I suggested we share it, but in the end saying 'there's always room for dessert' I ate it all by myself.