"Cartridge" is a bit of a misleading term, I expect. If they're smart they'll go with SD cards, which are about as cheap as blu-rays per gigabyte nowadays while obviously offering much better read and write times (console blu-rays don't allow writing at all). If they want cross-platform games on the NX the cards will need to be at least 50 GB, so 64 GB would be an obvious choice. As well as saving money in production costs, going with SD cards would also allow players to buy their own and easily install games downloaded from the Nintendo store, like on the 3DS. But they could also use a proprietary format - maybe something backwards-compatible with DS/3DS cards, which wouldn't be terrible.