Smaller is better because it means I have more desk space for a bigger monitor, surround sound system;
smaller is better because it means more sleek and streamlined appearances, and greater optimal use of real estate for productivity;
smaller is better because it helps to emphasize the importance of the content you can output to your monitor than the computer (much like a small tiny camera versus an overwhelming DSLR pointed at you);
and, smaller is better because it is one step into the future in the application of miniaturization, and thus setting the objectives of the industry higher and farther to better and greater than present day achievements.
I love my Gigabyte Brix GB-BXi7G3-760 for these reasons (less the first reason though), and it encourages me to spread the word and try sell to people less powerful (but still powerful) and smaller, power saving units. If one could take one of these back 15-20 years ago, everyone would be aweshocked.
Did I mention that it is super easy for me to take this kind of horsepower with me on international flights due to its size? That would be something that you couldn't do so easily 15 years ago!
The equipment in this drawing would allow me to experiment with clustered computing for Adobe products and perhaps play Far Cry 4 at maximum settings on 1920x1200
Smaller is better because I hate wasted space i.e. I don't need 15 bays when I have 2 hdds. Easier to put in the entertainment center in the living room or take up less desk space in the office. Easier to move and transport.
Smaller is better because progress marches on and gives us the same processing power in a package a fraction of the size previously required. It's a physical expression of the triumph of man over math!
By putting a fast CPU and graphics cards in a smaller box you radically reduce the distance the electrons have to flow. Therefore allowing far more speed from the same components. Due to the tight confines of a small mITX case, air is heated more thoroughly and quickly, producing high speed convection currents and self cooling benefits. Or something. Gimme.
I really do want to build a mITX build next time. I have a full ATX Phenom that needs to move to my sister's house. Pity my poor, starving sister running Windows XP on an Athlon X2. I built that one too.