The difference is that when you load from floppies, (a) you have nothing else loaded and eating memory, and (b) from floppy, the game is loaded little bits at a time.
Piru's suggestions will save you some memory. If you still don't have enough then try disabling the PRELOAD option in the tooltypes of the game or the WHDLoad config file. It will stop WHDLoad from loading the whole game at once, which might allow it to run.
Failing that, Piru is right - you will need at least a RAM card, or better yet an accelerator with RAM on board.
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moto