I think I see now why so much of the left despises this sort of transition while praising gender transition to the moon.
Getting fitter, improving yourself, is all self-directed. No one can do it for you. Other people can help a little but really it’s all on you; you lift the weight or you do not. You lose the gut or you don’t. It takes a lot of effort over time and there’s very little chance it’ll occur if you do not have motivation and will. It’s as individual as a thing can be in this world.
Gender transition, on the other hand, is something everyone else must agree on: this person is now a man/woman and if you say anything else, you’re obviously bad and evil. The left’s views on trans issues is all about being able to constrain someone to hewing to their ideology in a communitarian manner and to punish them if they dissent1.
It has eluded me until today why the left utterly despises people who get fit, but I think the above insight is a large part of it. The hatred is all because that result is an unauthorized, individual-focused and individual-driven upgrade that no one else has to consent to or agree on. And that it then increases your status enormously in society makes it all the worse (from the left’s perspective).