My inverse navel was snug to a 12mm barbell when it was first pierced (due to swelling). It is now pretty snug with a 10mm; certainly couldn't go any smaller. I do think it migrated a little though. Top one was also snug to a 12mm bar when initially pierced (due to swelling), but now is snug with an 8mm (as I mentioned earlier) and hasn't migrated at all.

Maybe it is just the angle, but it looks like you have a shorter barbell in the bottom? If they're the same length, it does look like migration may have occured.