I fully admit I'm still low level, so I don't have the experience you do. Still, the world (if not every single dungeon) is HUGE. I don't pick up much stuff at all -- I don't fast travel as a rule so I often will do two or three "dungeons" before returning to town and I plain can't carry that much. So I still think, even at high levels, I'll disagree. I mean...once you're level 40, are people really picking up tools that are worth like 1 or 2 gold a piece?
Exactly. At lower levels you'd grab everything to sell, at higher levels you have to stop to pick and choose. With a copy/paste/lower level dungeons that makes it seem bigger/longer than it would. Without that extra junk it becomes a two minute run. That's why there are already mods to add even more junk, it's filler some use it to role-play some use it to sell, some modders will make it useful, the game company gets to add a few hours to game play time and by throwing it around in a different order can make identical spaces seem different.
Some said they made the walk/run speed slower than Oblivian/Morrowind (can't say didn't play them) that gives the feeling of an even larger world. A trick Beth thankfully avoided was leaving 90% of the caves/camps off the map. Gothic III seems even larger by doing that.