To be fair the mac pro went through its own horrible shrinkage phase where it was not much larger than my $50 air filter. The old aluminum case used to be pretty large and pretty spacious internally, with many extra bays waiting for you to give them purpose. Not sure how the newest one stacks up to the first mac pro/mac g5 case.
I blame Apple's product line branding for this confusion. I make the same mistake all the time.