Note that 256 megs is how much physical RAM the iPad 1 has; the amount available to the foreground application is much less than that. Indeed, on the iPad 1 you're lucky if you have much more than 100 Megs to yourself while you're running.

And, as others have said, higher color depths will make some difference, and no doubt the port puts a lot less effort into RAM conservation than the original did.

On the other hand, I think that ran in much lower resolution and in 8bit color.