Who said there are not small parts only small actors?

There are no small parts, as in each character written has a purpose and you must play said character accordingly. Your part is only as important as you make it. A "small" actor may see their bit part as insulting, where a good actor would take the opportunity to play the part to best of their ability, to be noticed in their own way if appropriate, sometimes a character's importance is in their subtlety alone, their anonymity. Even as a chorus member or an extra you are still there to set an ambiance, to add to the show and therefore you should be in character and always doing your best.