Standard PSX memory cards have 15 blocks of space on them, each of which consist of about 8 kilobytes of memory. Therefore, the total usable space on a standard PSX memory card is about 120 kilobytes (it is often advertised that a PSX memory card is 128k in size, but the first 8k of space is used by the console to format the card).
Like discussed several times before, you must save PSX games on PSX cards and PS2 games on PS2 cards. Simple as that. Even if they were backward-compatible, standard PSX cards don\'t even have the room to save ONE game in an average PS2 title. For example, NHL 2k1 uses 2MB on a memory card....OUCH!