for manga that's a WILD amount of variety. not because that's OK, just because the industry is stuffed full of same samey sameness

I think minute differences was the point.

I genuienly don't believe you.

Either you are mistaken, or you likely have some kind of genuinely concerning medical condition.

Like I'm not trying to dox you, but take a picture of yourself, wearing fairly tight clothes, in the same pose as the anime figure, line them up at the shoulders, and overlay them and compare the outlines.

Use a camera that is further away but also zoomed in a good amount, and if you do actually want to post the image, blur out your face and any tattoos or scars... totally fine if you want to just keep such an image to yourself.

What you will likely find is that you are mistaken, you misjudged the proportions, or perhaps you are a skinnier lankier dude, but not literally the same limb thicknesses as the anime figure, or, perhaps you actually are that skinny, in which case, if you are past puberty, you likely have a relatively rare medical condition like chrones or muscular dystrophy, or anorexia, or are malnourished.

I've done a fair amount of karate and met a fair amount of Japanese people, met them as visiting teachers or competitors.

On average, do they maybe have slightly less body fat than other peoples?

Sure, to a noticeable but small degree, but not to the point that they all literally have the proportions of anime characters.

And my 'sample' of such people is of course biased towards those who are athletic, obviously not every Japanese person is a karateka.

Are there skinnier models that tend to be in a lot of magazines, marketing, advertising?

Again yes, but such people are not close to average, normal people, just the same as models in most other cultures with mass marketing, or actors in movies.