Yayyyyy //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
If by the right size you mean correct diameter for the sub (10", 12", etc) then a smaller airspace than the sub's recommended airspace will act like firmer suspension behind the sub - the air behind it will compress more quickly than if there were more air, which means it will be harder to make the sub move as much, and consequently get as loud as if it had a bigger box. However, this also means you can run more power to the sub safely, since there is more "damping" behind it.
Does this make sense?