Nah, you don't understand. Think about any of the Arma 2 maps. How many buildings are enterable? Maybe 20-25% on each. Except Takistan and Zargabad, they have maybe 90% of the building you can enter. Now Chernarus. Yes, that's a good map i know that. But try to do urban combat in that. It's nearly impossible. If you're stuck on one of the streets, cut off from the front and behind, you don't really have a chance to try and escape left or right because you can't enter most of the buildings. Now look at Altis. There's maybe like 5% of the buildings you can't enter.
So just think about it. Let's say you want to play mapX, it looks great, it's buildings are enterable, 20x20km, filesize is ..dunno 800mb but you have to have 10GB of files that you don't use (Takistan and Zargabad maybe) because of the reasons i listed above.
Now do the math. You have 10 gigs of files, out of that 10 Zargabad and Takistan is like 2. Yeah, make it 2 gigs. Now mapX for 800mb. That's 3 maps for 10.8GB. Yeah, surely not a waste of space and a waste of time in downloading.
Now let's say you download 5 standalone maps. MapA-E. A=1.2, B=1.8, C=0.8, D=2.3, E=1.5. It's 7.6 if i'm correct. 7.6GB of maps that you can have proper CQB.
And don't get me wrong, Arma2 maps were great at their time. Back in 2009 you couldn't even find a game like that. But that was 7 years ago. Arma has come a long way, it improved a lot. Like i said, it's the time of Arma 3 now. It's versatility is unmatched.
So yeah, just think about all i wrote for a second.