Modern flash carts like EasyFlash and clones allow for absolutely cavernous cartridge images. As good examples, see the C64 ports of Prince of Persia and Eye of the Beholder, which run entirely from massive cartridge ROMs.

Eye of the Beholder is about 1MB, for comparison Terminator 2 on cartridge from back in the day was 512kB.

But the cartridges themselves contains gigabytes as you say.