There seems to be a difference here - how much money you earn isn't perceived as entirely a matter of choice, or at any rate there will be a significant and unavoidable lead time between deciding to earn more and actually earning more.

Whereas body shape is within our immediate sphere of control: if we eat less and work out more, we'll weigh less and bulk up muscle mass, with results expected within days to weeks.

When I say "I can move my arm if I want", this is readily demonstrated by moving my arm. Is this the same sense of "want" that people have in mind when they say "I want to eat less" or "I want to quit smoking"?

The distinction that seems to appear here is between volition - making use of the connection between our brains and our various actuators - and preference - the model we use to evaluate whether an imagined state of the world is more desirable than another. We conflate both in the term "want".

We are often quite confused as to what volitions will bring about states of the world that agree with our preferences. (How many times have you heard "That's not what I wanted to say/write"?)