I think it has to do with the geist glyph because I have that glyph and mine is huge when I empower him.
It has nothing to do with the geist glyph. We have 2 Unholy dk's in our raid, myself and another. He does not use geist while I do, but both of our Timmy's stay huge.
It's just a simple bug, if you noticed before this patch they got huge for just a moment and then shrank, some bug is just preventing the shrink.