Looking amazing! I love it!!
Simple - I'm not skilled enough yet to do anything smaller than a SOIC-8As long as you're going surface mount, why not switch to a single inverter package? Five pins I think although one is unused. It would be even smaller and might cost a bit less.
Limitations for assembly is always a good reason. I designed some SIMMs with four chips on each side, even though there was room for all eight on one side, because four & four left room between them for a soldering pencil...Simple - I'm not skilled enough yet to do anything smaller than a SOIC-8
The other reason is that the MAX660 SOIC-8 part is pretty much the least expensive we can get while not being crazy small to solder. There's one from TI that's a few pennies smaller but it's a tiny 5-pin package.