Like you open a door or window. The sound blooms and is not bouncing all over killing itself. Play the same rig in a small sealed room, then go outside or to a large venue and play the same rig. Generally it will sound brighter and less boomy, but the full range will stil be present. The lows that are resonating all through the solid objects and walls of the small room, will still be there in the large space, but only get heard once or twice per note. Think of how sound waves replicate and bounce around, and how low range travels further. At a certain point of sound space, the highs will begin to bloom, where the bouncing lows are not replicating in your ears.
You can simulate this effect in a small properly treated room, with certain freq. cuts/boosts, depeding on the space and gear used. Generally resulting in what would be described as a brighter tone, but lacking in bass response.
