CD-ROM gave you two orders of magnitude more storage.
The largest cartridge released during the Genesis/Mega Drive's heyday was only 5 MiB and most were a fair bit smaller than that. The library is very much a mixed bag, but I think a large part of that was game developers needing time to figure out what to do with all the extra space. 512 KiB dedicated to the second 68K, 256KiB that could be swapped between the two processors or split into two 128 KiB banks with each processor getting one of those and 64 KiB of PCM RAM (there's also a little RAM for the CD-ROM drive's buffer and some battery backed up RAM for saves, but those aren't directly useful for game assets).
The Sega CD actually had a fair bit of RAM in it for the time.