Welcome. Might add that opening the backup zip and looking for large files may not be the only issue. If you do temporarily remove a large file from course_files don't forget to edit the related .xml files before re-zipping.
The backups could contain references to mods/blocks that were add-ons to the 1.9 ... no longer compat with 2.
As a fall back, consider creating 2 file system repos in /moodledata/repository/ One called 19data. In there, copy all the 1.9 course ID folders and clean them up ... ie, remove moddata, backupdata. Any time a 1.9 restored course has issues with files, at least one can set that course to use /moodledata/repository/19data/[oldcourseid] to re-acquire files. Can also use file system repo for all your 1.9 backups in a 19backups folder. Only use them as repo's in courses when you need to. I sometimes use that 19backups folder as a 'work/looksee' area before restoring or linking via file system repo.
Yes, that does eat up some serious HD space but once one gets all the courses 'settled into the 2', they can be removed manually - with one cravat ... users, if having to re-link to files in their 2, should NOT use 'alias' when selecting a file from a file system repo.
'spirit of sharing', Ken