The cartridge interface is general-purpose, so you can stick whatever electronics you like in the cartridge. If I was producing these at scale I'd burn into an asic the compressed data, the decompression logic, and some paging/addressing logic. So you say "give me article 1234" to the cart and it streams you the decompressed article. You could do that with an fpga if you were making them in low volume.

For an example of the crazy things you can do with gameboy cartridges look at Gameboy Camera.

The official Nintendo cartridges never used more than 4 megabit of ROM. But many unofficial "backup solutions" integrated a microSD reader and had quite a bit more space available.