For me with Pro 2.7 (albeit upgraded several times from something much older) the discrepancy ratio of reported index size to actual storage usage is 70MB to 48 GB, or to use a single common unit, 7:480!

Additionally, using the supplied button and deleting the Index does not remove the thumbnails.
Update: It took 47 minutes to delete the thumbnails manually:
@setlocal enabledelayedexpansion
@echo --- Started: !date! !time!
pushd %appdata%\..\Local\ESRI\Index
rd /s/q Thumbnail
popd
@echo --- Finished: !date! !time!