Wrong math, though...
Because there are 4 channels for every block and that space would (I suspect) need to be allocated for each possible instance of each possible block in the preset.
If my count is correct from this page, that's 99 block instances for Axe Fx III.
So (rounding to 100 for simple math), 1 preset would potentially be 100 * 4 * 10 == 4000 bytes.
Now multiply that by 1024 presets and you get 4,096,000 bytes!
Edit:
To be clear, this is speculation on my part... I have no inside knowledge of the internal preset format, but I think this is how it would have to work.