alright, i just exported 2 levels for testing level to level transition... i then realised that both standalone levels are nearly equally large... My "complete" city level,which uses tons of buildings, plants and what not has a volume of 923 MB, while the second one, a small safehouse (basically 1 room with some furniture, not much really) is at 909 MB ...
so i got curious at how those both could be nearly equally huge, so i went through the files folders of both stand alones...
solution: in many cases (funny enough not all) it seems like the standalone version has ALL entities from the GG entity folder in it, or (i gotta doublecheck) the small level has all entities from the big one in it...
i tend to believe the latter, because it appeared to me that if you switch between different levels in editor (means, you work on one, save it, load another, work on it, and so forth) , that GG doesn´t clean the memory, which means it takes over data from your previous work... it became most obvious to me when i loded a flat new level and for some reason the terrain textures of my previous work were still there, even after doing "new flat" several times...
also after a long day of working on several things, the memory of my PC seems to be under quite some load...
so, all in all i guess memory management is not really optimal... i´m really no expert, so all i can say is based on my observation and guessing