First of all, the databases sizes:
vBulletin: 4,8 GB
XenForo: 10,1 GB
Of course I wanted to find out why there's such a big difference. So I checked the post and thread tables:
vBulletin threads & posts: 62,1 MB - 3,7 GB
XenForo threads & posts: 40,2 MB - 2,0 GB
Is that possible, or did something go wrong during the import that caused a loss of data here?
Then to find out why the XF database takes up so much more space, I saw that my vBulletin was actually using Fulltext search, but can it be that I have a 6,8 GB search index in XF for roughly 5.5 million posts? Because it sounds high. If so, a full text functionality or something similar would be very welcome.