Sometimes it's useful to have a very small utility class that is used
solely within a larger class. They're so tightly coupled that it's
convenient to have them together in one file.

Sometimes you don't want to implement the utility class as an inner
class because inner classes can cause problems. Some obfuscators, for
example, don't handle them very well.

I would not put two large classes together in one file, nor would I
include a secondary class in a file if it were used anywhere outside
that file.