Hi Simon,
No, the upper values are fine. The minimum values are what's important. When we set up the xy domain values, we use what we call the 'horizon' aka valid area of use for the coordinate reference system. That's often much larger than the actual area of...use/interest. For example, the area of use for GDA94 MGA 55 is a 6 degree wide zone between 144E and 150E and 44S and 12S (very roughly!). The minimum values for the horizon for transverse Mercator with those parameters extends to the south pole and 45 degrees west of 147E. Geodatabase storage uses approximately the minimum horizon values and then calculates the maximum values by using the minimum values, xy resolution value, and the max integer storage, 2e53 - 2 or 9007199254740990.
Anyway, what's the actual feature class extents? And they might be messed up compared to the actual data in the feature class. I would also try rebuilding the spatial index.
Melita