Let me rephrase that sentence:
If you'll look on TF2 maps you'll see the areas are quite big, even inside buildings.
The areas are not always huge, but they are bigger than real life areas. I ussualy like to put a Heavy.mdl in a map and check with him in sight how big/small a room is. If I can't throw in a room about 5 Heavys then, according to me, the room is too small. But, of course, you can disregard my advice.