Hi Paul, MoI is a 32-bit application and so it's only able to use about 4GB or so of your total memory.

When you get that insufficient memory warning when exporting, that means that there is not enough room in that 4GB to hold all the data that you have currently asked to generate.

There are a few things you can do to solve this.

One is that you already mentioned - break it down into some smaller files instead of one huge monster file.

Another one is that if you're only running out of memory when generating polygon mesh data to go into a polygon mesh format, you can do just the export in smaller chunks at a time instead of all at once. To do that select some set of objects and then use the File > Export command instead of File > Save As. The export command only writes the selected objects to the file instead of everything and generating meshes for only some subset of objects instead of all at once should also reduce peak memory load.

Another thing that can help reduce memory load with more complex models is to adjust a setting to make the display mesh rougher. By default MoI generates a pretty high density display mesh which makes objects look nicer but it can also consume quite a bit of memory when you have a whole lot of model data. See this previous post for the settings to adjust to make the display mesh density rougher:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4558.4

- Michael