True .. but only up to certain points which most users probably won't encounter. Firstly, the individual *.img files need to be under 4GiB to satisfy the FAT32 file size limit (for FAT 16 that's 2Gib minus 1 byte). Then there is the amount of individual tiles that the unit can handle, in old models that's 2025, but new units can handle in excess of 4000 tiles. Neither the 4GiB or tile limitations are likely to be reached by most users with 'normal' maps.
The big stopper is usually the amount of individual MapIds which i think is around 16 or thereabouts. So if you were to load up a big card, say 32GiB, with lots of different small maps then not all will be seen by the unit.