Dont worry about that i dont think its a problem, just look at the size of this level, http://www.youtube.com/watch?v=zirtiRvNXh8&eurl=http://www.n4g.com/ps3/NewsPending-193782.aspx
The themometer is only half full (or is that half empty?), it seems to depend more on the amount of different assets and textures you use for example if you make a wood area using mostly using the same textures and object, although varying in sizes, it would use less of the themometer than one full of every texture, object etc etc in the game. As i said earlier all the in game levels are made within the same limits and some of them look pretty huge.
I think they said the usage goes up more if you use a lot of stuff captured with the Playstation Eye. I don't use the Eye much, but might for this. I imagine you can just load a jpeg full screen on PC or Laptop and take pictures of the screen. I don't think you can import content from XMB though.
rp2000