The PSX memory cards were 1 megabit (which is a nice round number).
The GameCube's MC59 (the grey memory card) is four times as big, and is 4 megabits (or half a megabyte). The blocks are supposed to be the same size as PSX blocks (but with an index of some sort taking up 5 blocks, instead of 1 block on the PSX).
The GameCube's MC251 (the black memory card) is four times as big again, and is 16 megabits (or two megabytes).
The PS2's 8 megabyte (64 megabit) memory card is another four times as big.
When Sony made the leap to a memory card that was sixty-four times the size of what they were using, Nintendo just thought (yes, I'm a psychic) that was a pointless waste of money, designed to give the PS2 more inflated statistics and make it look more impressive, without giving people something that they actually needed.
Game save data doesn't increase as you make the graphics better or add more FMV to a game.
But, developers will take anything you give them.
By having two 16 megabit cards, and two 4 megabit cards, you have about the equivalent of fourty PSX cards at your disposal. But, the makers of sports games don't think that's enough. You should go buy another MC251. Or pester Nintendo to come out with the next stage of cards (which would be the, most likely purple, 8 megabyte card). Your games will be more fun that way. You should thank them.
My brother (who has a PS2) was watching me play Metroid Prime back when it came out, and he was stunned when I pulled up the GameCube's memory manager screen. "How can the save possibly be only taking up one block? That doesn't seem possible. It's gotta get bigger as you go farther in the game or something."