Shut up 'a' your case

Sorry about the mrssage headline.Those old enough and fool enough to remember
Joe Dolce's dreafull single, "shut up 'a' your face" might acknowlege the poor attemt at humour.

Anyway, I'm just wondering about cases and which would be considered the quietest.
I just bought a computer here in the UK from scanuk who build computers for musicians.
I made a big mistake in not checking the dimensions of the case I asked for
(coolermaster RC100 Cosmos) a very nice case but way too big to fit in to it's designated slot. So I called them and they have agreed to have it returned and they'll put it in a smaller case for me.

Now coolermaster claim that the size of this case helps increase airflow thus making cooling more efficient and therfore giving the fans less work to do and so making it quieter. Seems logical enough I thought.
But, compared to my smaller case (not sure what make) it's hard to tell whether it's quieter or just that the fans sound lower pitched which I guess they would if they are not spinning so fast. But the difference is really too small and I was not happy as I was told that it was virtually silent.

So I started wondering if perhaps the larger cases don't actually act as a sound box and therefore amplify the fan noise? So it maybe true that the fans are not spinning as fast and therefore less noisey but this is countered by the fact that the larger box is amplifying them.
I put this to their engineer and he seemed both dismisive and confused at the same time.
What do you reckon? Do I need to get out more? :)

Steve