This is rather due to the ROM itself.
In the case of OSX that is the problem, but MorphOS only uses the first 128MB even if you're running on a Pegasos with a PC 9200.
One of the developers mentioned that unlike the 256MB 9600, the 9200 treats it memory like two distinct 128MB blocks.
While the MorphOS driver for the 9600 will use all 256MB (if present), the 9200-9250 driver won't.