The progression in size was basically motherlode -> "Big Fucking Motherlode" -> BFM 500, with the BFM being more or less the motherlode with a bottom and side pocket and slight redesign, and the BFM 500 being a
more significant redesign (ignore the tacticLoL website, they just give the actual dimensions).
They also made a 500 version of the original motherlode that seems to be
almost the best of both worlds, having the price/size of the motherlode or classic BFM with the improved big-pocket-on-bottom design of the BFM 500. The only downside being it sticks out further from your back an extra 4 inches compared to the classic BFM, although that can be solved easily with an added strap or just good packing.
The only thing would be the lack of a padded laptop pocket, but with the extra space I think I could get away with either getting or my own sleeve.