If I were given the job of making it smaller. . .
The width is determined by the widescreen LCD monitor. A non-widescreen monitor would shave some width, and most games don't use that extra width anyhow. You could build it around a 20-inch or 21.3-inch LCD with 4:3 ratio.
The cabinet depth is determined mostly by the large, protruding control panel. You can see they are using only about half of the CP space, so you could take out several inches there . . . . depending on what controls you actually want on yours, of course. (Some people I know would just cram in more controls, rather than make the CP smaller!)
The height is already quite short, so I wouldn't see anything to gain by trying to cut that down.
I have a quibble over the location of the speakers. It looks like they are under the CP, above the coin door, and the speakers are sort of aimed down toward the player's knees. I don't have ears on my knees, only grasshoppers have that.
