128 kB is actually a good lot of storage for the kind of things the FMC in a 737 would need. I would imagine that any data stored wouldn't be any more complicated than strings of ASCII text (Routes, etc), think about how much text you could fit in a simple txt file, probably hundreds of routes. 128kB would (very) roughly equate to about 128,000 characters, more than enough for what a FMC needs!