Is it causing a problem? Maybe overloading your automated backup systems you may have in place? (that is an issue I am addressing at the moment for my forum).
I will not profess to be a database expert, but in my vb database I see no huge search index, but I do in xf. I do know it has been disscussed at length a while back... but I have extremely limited internet connection at the moment so I cannot search at the moment.
does IPB use innodb table for that search table ? if not , then as Kier explained difference in disk space consumption is due to using innodb instead of myisam tables for xenforo. MyISAM tables used in vB compresses it's indexes while InnoDB doesn't. That's partially the reason why InnoDB tables are larger.
InnoDB tables can be between 50-300% larger in size than MyISAM tables.
You can test this out actually, just take a MyISAM table and convert it to InnoDB and note differences in sizes. Or take a InnoDB table and convert it to MyISAM and compare. You can convert via ALTER TABLE command