Companions CAN NOT have less than 50% of your XP. If they join your party with less than that, their XP jumps up to the 50% mark. So if you're level 20 (between 540,000 and 700,000 XP), Hirge will be between Level 16 and Level 17 (250,000 and 350,000). If you've been Level 20 for awhile, it most likely she'll be Level 17, and almost ready for Level 18. But that comes out to whatever your exact XP is at the moment she joins.
Companions in your party get slightly more XP than the 50% floor so your favorite Companion will have a slight edge in XP over a Companion you use once in awhile.