Indeed, Joss is probably doing the best he can.

Note, for instance that he asked Jewel Staite to fatten up for the role of Kaylee in Firefly, and she wasn't required to put it back on for Serenity, which is the reason she looks considerably thinner in the movie!

The sad thing is that it's the people with the purse-strings who want to keep TV homogeneous. It's only indie stuff like the aforementioned Guild which can dare to be different.