Frank, I guess I know what is happening then.
At some point Illustrator introduced Large Canvas mode (you can read about it in detail here: https://helpx.adobe.com/illustrator/using/large-sized-artwork.html).
This was done to allow users to work with larger dimensions directly without calculating the scale manually.

The way it works though is cumbersome. When a user creates a document and specify width or height larger than the hard limit (2^14 points: 16384 points, 227in / ~5766mm), Ai EMBEDS the hidden scaling factor into the document. Illustrator warns about is with a sign next to OK (see the image attached).

This scaling factor is fixed and can’t be changed or disabled later. Making a document smaller won’t magically remove the scaling factor and rescale the art back. This coefficient is respected by some PDF readers — when a document is viewed, but can be thrown away, if the artwork is imported instead — and this is what probably happens with you.
This is no mention in the FAQ (https://helpx.adobe.com/illustrator/kb/large-sized-canvas-troubleshooting.html), but I am not really surprised — even though it got introduced in 2020, the tech is still not perfect and here at UV dozen reports exist about it flawed behavior.

As for the moment the only way to convert a large canvas document into a normal one is to copy and past all the contents into a new one (with Paste Remembers Layers enabled, if needed).

Can you please check if that’s the case and comment back?

Hi Egor!
Thanks for getting back to me.
It's all at once rather than each element being scaled separately.
It seems as though this shouldn't be the expected behaviour though, as I would personally have expected a 1920x1080 canvas in Illustrator to be brought into After Effects at 1920x1080 (rather than 192x108). Perhaps the issue is with import into AE rather than export from Illustrator, but I would argue that there's certainly an issue to be addressed here.

Compounding the issue, even if I save off an artboard from Illustrator as an individual file the file retains something from the large size canvas originator that means that even the single artboard file imports at 1/10th scale. Is there any way of manually turning this off?

Thanks again for looking into this for me!
-f

All at once (which is expected, due to how large canvas is constructed), or each element get scaled separately?