It would matter if the next system uses cartridges.
I think you misunderstand the point I was trying to make. You were talking about the space used on DVD discs. I'm just saying that whether a game on a disc uses 512mb or 4.7gb or even a mere 64mb the cost to publish it is still exactly the same... because the cost of DVDs are fixed. Whether you use all the space on the DVD or not, it still costs exactly the same... you can leave most of the space blank, but you don't get any sort of discount in cost by doing that.
But with Carts its completely different. With carts it actually does matter how much space you use, because carts can have small memory or large memory depending on need. But even the smallest and least expensive cart you could possibly create is still going to cost way more than a 10 cent DVD, even if that cart only used 16mb of space...
If you think about it, all a DVD actually is is an extremely thin layer of aluminum sandwiched between layers of plastic. This makes optical discs EXTREMELY inexpensive compared to any other storage medium. Carts are MUCH more complex than this. I'm not quite sure all the components and materials needed to make a cart, but needless to say its quite a bit more complicated than a thin layer of foil surrounded by plastic.
Both mediums have their pros and cons. For optical medium their biggest plus is that they are extremely inexpensive, but that's about it. They are easily scratched, slow to read from, and cannot be written onto or saved onto the way carts can. I'm sure DS carts are far less expensive that N64 or SNES carts were back in the day, but the old argument is still applicable... just not quite to the same extent that it once was.