As a broad rule that sounds OK - literally, a "bit" often means a " small piece of", but it is not always used like that. It is actually possible to say in normal conversational English - " I'd like a bit of milk in my tea" etc (in the sens of " a small quantity of), but perhaps it is not strictly the best usage. I can imagine a recipe saying : add a little (small quantity of) grated cheese to the dish" , for example, but generally if it is something solid, " bit" does sound better - I'd like a bit of bread with cheese sounds right rather than "a little bread". Sorry if that is not helping you "understand it once and for all" ... sometimes you just have to get used to what sounds OK and what doesn't, by experience...