One, possibly extreme way of compressing data is to record only the steps/inputs used to create it. Something like what ModNation Racer does (curse those long loading times as it re-creates the track). (Google for videos of the track editor in ModNation Racer).
i.e. if i was storing some terrain, i'd store all the manipulations starting from a flat plain. i.e. apply the volcano shaped brush with this force, here. Apply the terrain smoothing operation over this area, with this force. Add my predefined object here. Using bezier-curve-like curves with 'key-frames' to construct an entire track/flight path/whatever.
Many other games do this sort of thing not just for level loading, but for gameplay too, by making the game deterministic, sending only player inputs (i think PixelJunk Shooter 2 does this).