Why is waveform of the voltage across the DC-link capacitor alternating?
You have a timeconstant of RLoad * (C1 + C2)  = 20us, how does that compare with your switching frequency (Which I make to be only about 600Hz)? 

C1,C2 need to be large enough to keep the timeconstant long compared to the period of a switching cycle otherwise you will the voltage across the caps changing significantly during a cycle, and as the load voltage is the +-(supply voltage - the voltage at the midpoint of the caps).... 

This stuff is easy to calculate, but I would hint that in such a converter the ripple current is likely to be as big a design factor as raw capacitance.