The issue with Asterion slimming down is entirely with the workload it would take, yeah. It makes sense in-universe, it would not be beyond Asterion's capacity. However, making another set of sprites (and clothes) for Asterion, then remaking the CGs would be a very big endeavor for us and we aren't in a position where we could really do that.
Reusing his previous sprites wouldn't be feasible, sadly. They are rigged in different ways so we can't just switch them back and forth, because the half-healed sprites also move the arms around independently.
Looking back we should have anticipated that some people would have preferred the half-healed/a slimmer look. I think it's a perfectly valid preference. We would do it if it was feasible given our resources — but, sadly, it isn't.