Also make sure you didn't accidental duplicate your terrain. thats probably your problems being you've doubled file size. Me thinks 
precisely why your mother warned you not to stare into it!Really nice.
But i find the sun a bit to bright.
All custom content goes into my map package. The cook screenshot above, that's all that happens, it doesn't create any file.
Ok, I just went in to an old save that didn't have any copy&pasted lights, added my own light (deleting the old one), typed the same settings (not copying & pasting the icon) and my map goes up to 874mb after build. There's a setting somewhere that is screwing this up. My old save doesn't have shadows so I'm assuming this has something to do with it. The only thing I did was untick bCastShadows on the terrain (which should of lowered the map size?)
aha. I swear that was only there because I hadn't been saving the maps after it went up to 800mb. I actually delete that and it's still 800mb. So I have bcastshadow off, 1 domi light, and still 800mb. Maybe it's just how big my map is with shadows. I guess I'll start work on an older build (1 day behind) and keep the old light which doesn't look as good.
Another thing, I tried to cook my map and all I get is this:
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How long does cooking usually take? Because this is over in a few seconds and there's no saved file anywhere.