I think you have a few factors here.
First, since neither manufacturer uses EN ratings there is no standard, and the ratings can be very different from different manufacturers.
Second, the specs on the two bags may be further apart than you suppose. I note that there is no girth dimension mentioned in your post for the golite bag and I couldn't find one on their website either - just a reference to a "thermally efficient taper, the Golite Mummy Fit" . Big Agnes bags tend to be wide - as the 67.5" shoulder and especially the 64" hip numbers confirm. If the golite bag is significantly tighter, say the size of a WM ultralite at 59" shoulder and 51" hip, then that makes a big difference in how much bag that down has to fill, plus makes a warmer bag by being tighter. Top bags are not 1/2 of a mummy bag - they are more like 3/4, since the insulation needs to come down the sides as well as over the top.
Add up all this stuff and then you can see that the difference isn't as big as it seems at first glance. So it's likely that there is nothing like twice as much down on top of you - the difference would be quite small.
If you could put your hands on both bags and measure the loft, that would be interesting info.