The GBA didn't have much RAM. There is a good chance tiny chunks of the game get decompressed as needed, and there is never a time when the whole thing is decompressed at once and can be dumped.
You don't need everything to be decompressed all at once to dump it. You could continuously dump the memory contents while exploring the game (or having a fuzzer do it for you).
Comments
The GBA didn't have much RAM. There is a good chance tiny chunks of the game get decompressed as needed, and there is never a time when the whole thing is decompressed at once and can be dumped.
You don't need everything to be decompressed all at once to dump it. You could continuously dump the memory contents while exploring the game (or having a fuzzer do it for you).