Two suggestions...
1. I would definitely go down on the rodent size if it was that noticeable for that long. When I feed, food lumps are usually gone in two days, definitely gone after three...most of the time there is a major change even after the first day to day and a half.
That said though, it might be helpful if you could post a picture of your animal with something in it for size reference...a nickel or quarter maybe? I'm just being a little confused at a hopper mouse being too big for a six month old brb for some reason.
2. I would bump up the hot spot to closer to 85f, even if not all the time, during the daytime to help with digesting meals...you can get a decent on/off timer at walmart for just a few dollars and put the heat on a 12 hour cycle to be on during the daytime and off at night.
One other thing I'll just throw out there...personally I believe in the idea that less hair/fur, equates to being easier to digest. If your animal will eat (some may not) and you have access to rat pinkys, I would consider trying that instead and see if that seems to help the digestion process a little also.
Again, my experience with brb's is limited, so if Cliff, Dave or one of the others here who are much more knowledgeable and experienced think I'm off some, I would definitely lean in the direction of what they suggest instead of what I've told you.
Charlie