I had the same issue, with similar size ~380MB creative world, generated simple without any downloaded content in the Minecraft.
For me the issue happened (as I remember) when the world was first loaded in 1.6.1 or in 1.6.0, not sure. (when the game was restarted afterwards immediately, the version was 1.6.1)
I don't know if this is related, but during load 2 resource packs was also downloaded (or loaded), according to a pop-up with a progress bar.
Volner, Csaba
I had the same issue, with similar size ~380MB creative world, generated simple without any downloaded content in the Minecraft.
For me the issue happened (as I remember) when the world was first loaded in 1.6.1 or in 1.6.0, not sure. (when the game was restarted afterwards immediately, the version was 1.6.1)
I don't know if this is related, but during load 2 resource packs was also downloaded (or loaded), according to a pop-up with a progress bar.