Indoors Help

Can I see some pictures of the space?

Probably your biggest problem is you're making a box and dividing it into smaller pieces and/or shoving in big props to break it up. That's a no-no. Start like you would with an outdoor area: being with a small building or area, then go with the flow from there. Drawing it out on paper helps for some, but not all; find what's best for you.

First of all, fullbright almost always messes up the perspective, as well as your retinas, make sure you have lights.

Otherwise, what i've found in regards to this, is it's often learning through first hand experience. Some things in mapping are very hard to be taught, scaling one of them. People can say "This are is to big" or "It's cramped here", but you really need to work it out from your own examination of gameplay. I can see in your screenshots some rooms that are to big, as well as other problems. Some of these such as the gigantic killing maze feeling could be solved with signs. Others such as the feel of the huge room could be broken up with hight variations, interesting brush work (don't go over the top though, just keep it from being all cubes and more cubes) and some areas being broken up by props.

Apart from this, a lot of what you need to learn about scaling is from practice. Make your map, run around, test it in gamedays, very importantly listen to people's suggestions (the have lots of experience they are willing to give to you) and closely examine gameplay (ask questions such as "Is one class overpowered in this area?", "Is this area used to much?", "This area to little?", "Are snipers to overpowered?", "can sentries lock down this room?" etc.) and from this, you can deduce what you've done right and what you've done wrong. You're not going to make a map that's going to get official first go. It only can be done, like most things, with lots and lots of practice, and more importantly, looking at your past mistakes and avoiding them into the future.

It's also an optical illusion. Spaces outside aren't always bigger than spaces inside, but the open sky/skybox always make them feel larger.

Of course it's easier to make good rocket jumping space outside because the skybox can be really big and doesn't feel weird as it's not visibly way above you.

Depends on what you're going for too. Look at Well's last point, it's a huge room, but it's got a control tower, gigantic rocket. Doesn't feel cramped because you can double sticky and not hit the ceiling, but doesn't feel oversized because it's a rocket base.