Really odd.
I just tested with two hour timeline, simple cuts, one audio track. With 25 frame handles .omf file was about 1.3 gigs.
Last summer I did one OMF file with this Copy Complete Audio Files
MediaFiles folder was over 5 gigs.
Sound engineer did a little more handwork, I don’t know what, but managed to open OMF successfully in ProTools.
So it is possible also with External Media Files.