The size is because you included .cache files (just delete them before creating rar). Also I don't think its necessary to include maps that are in developement (not part of the demo at all). Somehow when I tried to check maps through debug I even stumbled upon La Caza's first level.
I couldn't play through everything (cos I'm lazy and impatient) and I want to got to sleep soon so I will only point a "few" things out about the technical stuff of levels.
1. At 00_example.map. Have you ever wondered why fps drops for a short time after loading it? Thousands of books dropping to ground (there might really be a thousand) can be quite heavy for computers. Fix would be just not to rush like you seem to have done and place the books on the ground manually (I don't mind though since this is first map and a demo after all).
2. Minor thing, but there are a lot of walls and other stuff that just can't be seen and as so, could be deleted. Eventually these pieces raise the file size... I gotta say I found some from frictionals maps too!
3. In the Intro.map (outside) you can see a lot of white dots through the rocks and you could have saved time and give it some aesthetic boost by using some normal rocks with different scaling. Spamming one model gives area unnatural and repetitive look, obviously. Also there's 33 sewerduct_waters... And yeah, they look unnatural (good idea, but I guess hard to implement) ^^
4. Tone down them billboards! Its obvious that you don't want to pay too much attention into them, but do pretty bad looking billboards make areas nicer or break the experience? Would it be better to drop them altogether? I'm not saying delete and redo all of your billboards now. Just remember these when you do more of them and ofc: to improve, one must try and experiment as much as possible.
One good tip on creating lighting and billboards is:
Tinker with every color! Every value as 1 gives the areas
huge contrast between pitch black and looks... ...yet again unnatural
5. Why do I spam about unnatural? I think that if everything is
too weird and out of place, player cant feel immersed.
Keep on working with the editor, observe how frictional made their maps and be creative to improve your maps aesthetics! I hope you take feedback in the right way to keep you rolling and learning.