My cold take on this is that any sort of idiom that enforces a normative standard for bodies is inherently ableist and thus based on that fact alone it's bad (but it's also bad in other ways too, for example being cisnormative).

Am I going to argue my case for this outside of an essay or a discussion with people who are on the same level? Not a chance. It'd never land.

But my angle is that disabled bodies very often exist outside of the norm for a variety of reasons, whether it's because of physical reasons like amputation, congenital disorders, muscular dystrophy or like medications that mess with metabolism or things that disrupt the endocrine system or even just being so damn depressed that you either stop eating or you can't get out of bed so you aren't burning many calories.

Is ths idiom intentionally perpetuating ableism? Maybe, but not necessarily.
Is it overtly engaging in ableist rhetoric? Possibly, but it's quite likely not as well.

And this is why I don't think I could be bothered litigating this with a person who doesn't get it.

I just don't have the energy to have someone say "No, but it's not ableist because it doesn't say anything about disabled people directly" and to go through loop after loop of dialogue where it's like:

"Is a driver responsible for driving drunk and killing someone, even if they never set the intention to do so"
"Yes, you have to take responsibility for the consequences of your choices"
"If the consequence of your choice is that you pathologise or denigrate disabled bodies, does it matter what your intention is?"
"No, because it's still the consequence"
"So therefore this idiom is ableist?"
"Well no, because this doesn't say anything about disabled bodies"

Or

"Speaking in terms of consequence, is it any different to invite all but one of your friends to dinner and to invite everyone to dinner but knowing that one friend is coeliac and the restaurant doesn't cater to gluten free dietary requirements?"
"No, it's still exclusion"
"Does it matter whether you explicitly exclude some people from being considered part of the entire spectrum of normal or whether you only do it implicitly?"
"No, because it still excludes people"
"So using an idiom that excludes some bodies from the entire spectrum of normal is ableist?"
"Yes"
"Then by definition that idiom is ableist, correct?"
"No, but it isn't explicitly intended to exclude anyone who is disabled so that means it's not ableist"

So often it doesn't even matter how gently I approach these discussions or how much I soften the blow by assuring the other person that I'm not going to rake them over the coals with accusations of them being ableist or how often I tell them that I'm only interested in discussing the nature and function of the idiom, it just goes around and around only to end at the same denial.

I can either pursue this stuff or I can maintain a sense of hope for humanity but I don't get to choose both. (Though I am autistic so it's not uncommon for me to struggle with getting my tone right for allistic people.)

It's just frustrating because this stuff poses no threat to that type of person, it causes them no harm, and despite me personally being the one who is on the receiving end of this equation I'm still offering to gently and patiently midwife the whole process so that we can reach a logically consistent conclusion and maybe even agree to acknowledge that the idiom is unintentionally ableist before shaking hands and going out separate ways with the hope that they'll maybe spend a bit of time reflecting on how their words can affect others at some point. But nah, that would be asking too much from the other person.

I really wish I had more gusto for talking people around on matters like this one.