wp_usermeta is HUGE, am I missing something in my DB maintainance ?

Probably a noob issue, but I cant find it via other searches... I'm new in working with an existing WPMU site, and the wp_usermeta is HUGE (3.5M rows for 1200 users), am I missing something in my DB maintenance ?

This came to light as we prepared to migrate to a new server, and I found that all the backups and plesk migration stuff fails on the dumps on a couple sites with these huge (this is the biggest) tables...

Niel