After exporting a multiple-art board illustrator file to individual illustrator files, the new files import to after effects at 10% of the correct size.

It is the exact same issue as described in detail here: https://community.adobe.com/t5/after-effects-discussions/illustrator-files-importing-at-10-size/td-p/11464768

There would appear to be a problem with exporting from large-sized canvas to individual artboards, which can't be corrected from within illustrator. Currently the only fix would appear to be copying and pasting the contents of the illustrator document to a new file.