One reason for putting multiple related classes in one file is so that the poor bastard who uses your API doesn't have to spend half a day typing import declaration boilerplate and the poor bastard who has to maintain the code doesn't have to spend half a day scrolling through import declaration boilerplate. My rule of thumb is that multiple classes belong in the same file if you would almost always use a large subset of them at the same time instead of just one at a time.
