SF2's manual fits in a standard case, but there's no doubt the game was packaged in a double case for various marketing reasons:
1) it's a ROM-RAM HuCard like Populous, which makes it a special card. You don't sell a special card in a puny jewel case.
2) you don't sell the best port of Street Fighter 2 Dash in a puny jewel case.
Does "BRAM" stand for "Backup RAM" ?
Yes, but it's just a common abbreviation
The official but rarely used designation of the internal BRAM was the Ten no Koe (the "Voice From Heaven"), which is how the BRAM is called in the Momotaro Densetsu series.
As for the external back-up units, here's a quick translation of my French FAQ on Necstasy. It's not authoritative though, I don't own all those units and I might be wrong on their details. I'm probably a wee bit off-topic now, but here it is anyway:
o Ten no Koe 2 (Hudson) : requires one R6 (AA) battery.
o Back-Up Booster 1 (NEC) : similar to the TK2, but includes the AV Booster (very useful for the white PC Engine).
o Back-Up Booster 2 (NEC) : similar to the BB1, but includes a built-in NiCad battery.
o Back-Up Unit (NEC) : similar to the BB2, but specifically designed for the ShuttleGrafx.
I just squeezed the center of my Populous CD and I think I broke it!
The "Squeeze Your Populous CD" campaign just took its first toll :/