Definite volume refers to something which has a consistent, unchanging volume, which in practical terms may mean a static mass and density, or a predetermined shape, either of which will dictate volume. A cube with side length l has a definite volume because it can only be exactly one size.
One cup of raw plantains or one medium sized raw plantain contains: ~180-200 calories, 0.5 g total fat, 47-50 g carbohydrates, 3.5 g dietary fiber, 22 g sugar and 2 g protein. Because of it's high carbohydrate content, you need to monitor your portion, otherwise your blood sugars will spike.
Plantain, the fruit (of the Musa genus), is high in starch and relatively high in sugar, so it should be strictly limited in a rabbit's diet or avoided entirely. The House Rabbit Society recommends no more than 2 tablespoons of fruit daily for a normal, healthy 6 pound rabbit.