You don't mod UT3 to be more scaleable. The proper course of action is to mod UT2004 to be more like UT3. Remove the dodge-jump and make some slightly smaller maps and you're halfway there and on an engine anyone can run. 2ez.
As for the UT2004 demo, it had more than three maps for two gametypes. Having ONS in there was probably a big factor to its success (considering cheapskates are
still playing it). Epic not including Warfare was a huge mistake, especially considering they did it for a
really stupid reason. Unfathomably stupid, even, since Warfare is nothing like the epic journey across a virtual world like they'd promised.
Another thing is it included some decent maps whereas UT3 pretty much had the bottom of the barrel stuff for the respective gametypes plus no Warfare and no CTF. Sure, they could have put something crappy like Irondust in UT2004's demo, but instead they chose Rankin, a map that's very good for FFA and fairly good for TDM/1v1. It had staying power and is still popular.
Okay, yeah, we're all tired of Rankin, but think back to four years ago to before everyone was so biased against it. If you can remember, at that time Rankin seemed effing awesome -- people were claiming Hourences was the savior of UT DM.
What did people have to say about the UT3 demo in comparison? Boo, hiss, crappy, broken, stupid, terrible UI, server browser sucks, can't connect to any servers, GameSpy is ghey, no players, no CTF, no Warfare, bad maps, exploits, mouse input is screwed up, INI changes aren't sticking, runs like poop, bad player scale... Uninstalled! And then nothing changed for the retail release! :x