Actually, you could say the DS slot supports bigger storage BECAUSE it is slower, for the speed of the GBA slot is due to the memory being adressed directly, much like the built-in RAM, limiting the adress range to that which the processor provides, which corresponds to 32 Megabytes. This is also the reason why flashcarts using RAM (which is just about every SD- or NAND-loading GBA cart as well as most GBA expansion Packs) can double as a memory expansion.
The DSi might be able to support GBA emulation/virtualization when DSi Mode is cracked due to its larger RAM, but constraints on ROM sizes are very possible.