Yo in regards to this question, in my experiance a sub has a specific cubed foot of psace it should be enclosed in to make the siound the best, if you go above that size with larger boxes you lose sound quality due to the altered echo times etc, most likely the competition cars with overly sized boxes are there only for show, and there is a possibilty of them doing the box in a box setup where they get a box with the right cubed ft for the sub, then they put that box inside a bigger box (doesnt have to be a box can be anything that makes it look nice outside) so that they can show off, hope this helps ya out man.
the size of the box can change the peak of some subs like a 1' cubio box may peak at 154 @ 100 hz you go a lil bigger it may peak a 153.2 @ 75 hz it all depends on what you want and then if you make a snail shell box it will be big loud and deep but they are a pain and hard to make but typically you need to stay with in the recomendations of the manufacturer of the sub unless your after something specific.