I prefer building small cases for aesthetics, but I also figure there is less material waste in both production and recycling of smaller parts. It's sometimes hard to find nice parts in small form factor, so this is refreshing!
When I was younger I loved big, sexy cases with massive expansion capabilities, even though I rarely made use of them. Now that I'm older I focus more on efficiency, and large cases represent a huge waste of space. Better to tuck all that goodness into a small, well-designed case than waste my desk real estate on techie epeen. I've been excited to build a new mITX system for some time, but have held off for financial reasons. Give me some parts to get me started and I'll take the leap.
Smaller is better with a gaming PC because it's not only easier to build a portable gaming rig that you can throw into a carry bag, but you don't have to sacrifice anything in terms of speed for portability.