As an actual product available right now, there's this 250 watt inverter. from Enphase, intended to work with one solar panel. That's 54 cubic inches, or 12W/cubic inch. Google wants 50W/cubic inch, so Google is asking for 4x the power density.
This one happens to be configured for 48VDC input, but that's not hard to change. It exceeds the efficiency limit set by Google.
Enphase sells those little inverters for a one-inverter-per-solar-panel system, where power is combined on the AC side. The inverter, at 171 mm x 173 mm x 30 mm, is a lot smaller than the panel it sits behind. Making it smaller won't have any effect on system size.
One big difference: Enphase offers a 25 year warranty on that unit. Google only wants to run for 100 hours. They'll probably get something that will pass their tests but wouldn't last a year in a real solar installation.