If you were mashing up several bananas for a recipe, then you would eat more than if you were eating them whole. Several bananas mash down into a relatively small amount and if they were mixed into something else then you might eat a lot, whereas you probably wouldn't sit down and eat several bananas whole.
But if it is just one banana on toast, then I don't worry too much. After all, the rule doesn't say what you can slice it with, so I slice mine with a fork!