Ah! Knowing that, I bet that someone _could_ make a CJK font optimized for file size, which includes radicals by reference. It wouldn't work in every case, because radicals change size and strokes shift to make room for other strokes. In cases where the same radical appears in the same place, it seems like it would help.

But maybe that's not enough of a benefit unless there's also a way to say "okay you need to use this radical, but a bit narrower, but the strokes need to be the same width and not distorted".