Also the data on a cartridge is read using addresses just like memory, whereas on a disc you use tracks and then do a long sequential read. If you want one byte of data from a specific place you have to read much more than one byte from a disc, but from an addressed system you may be able to read an individual address then move on.

Also if you have 600mb to work with you tend to be a little less frugal thab if you have only a few megs to work with like on a GBA.