Possibility worth knowing.
Complete (split) machine ROM set zips:
64.6 GB
35,906 Files
Complete software list ROM set zips:
63.5 GB
100,157 Files, 511 Folders
Uncompressed unique machine ROMs:
127 GB (137,235,641,182 bytes)
135,660 Files
Uncompressed unique software list ROMs:
121 GB (130,190,962,439 bytes)
176,184 Files
For ball park figures (at 0.210) you can say machine and software list ROM sets are about the same size taking up around 64 GB each and compression of about 50%.
Not included any CHD just ROM.