Believe it or not, that's a thing, and it's called 'keistering'! Apparently, pro's can get a cellphone 'keistered'. But I think the flip-phone style, not an iPhone.

As far as microsd cards -- I think you'd be able to 'keister' many many terabytes. They are so tiny and so thin, I'd guess hundreds would fit.

There's a difference between "fit" and "fit without making it impossible to walk normally". And really, you'd want some sort of carrier for the cards, because otherwise they're likely to be lost or damaged. So it becomes a fairly complex question of the optimal balance among total carrier volume, gait effects, and maximizing internal volume of the carrier without thinning the walls so much they're likely to crumple. (Metal walls might serve, but then we're back to gait effects. Not comfortable.) And once the carrier design is in hand, there arises the question of how to maximize its bandwidth, which is itself a rather nontrivial packing problem.

Great, yet ANOTHER USB-C adaptor I'm going to have to pay Apple for.

A metric buttload.

The point was that the physical volume needed to store far more data is so small that you can fit it even the head without going to the lengths in movie, not that it'd be particularly pleasant. But certainly, smuggling things in body orificies has a long tradition.