I did consider that....
An extra change that you didnt mention is the need for a different return value. Currently .decompress() always returns a string. The new method in my patch returns a tuple containing the same string, and an integer specifying how many bytes were consumed from the input.
Overloading return values based on an optional parameter seems a little hairy to me, but I would be happy to change the patch if that is your preferred option.
I also considered (and rejected) the possibility of adding an optional max-size argument to .decompress() as you suggest, but raising an exception if this limit is exceeded. This avoids the need for an extra return value, but looses out on flexibility.