That's interesting, do you export with a top down ortho projection? I usually export at 1024 x 1024 resolution (a habit I've got from rendering texture maps at powers of two, but shouldn't matter I guess) with a MPD of 1 - and that usually gives me a file around 450mb, which boils down from to about 150 mb after a Meshlab delete dup verts and faces.
If I need a low poly preview, I'll drop the rez to 256. If I need more resolution, I'll tile the terrain, but usually in 1024 rez tiles.