Switching loss in a synchronous converter (buck, boost or buck-boost) when light load occurs
For purposes of this discussion there are three types of synchronous buck controllers/converters:


Those that do not have a light load PFM, pulse skipping, burst mode or diode emulation mode.  These will operate in CCM all the time, and when the load current doesn't exceed 1/2 the p-p ripple current the inductor current will go negative each cycle and you will have increased switching losses over one of the light load modes as Olin point out.


e.g. TPS54622


Those that do have some type of light load operation.


e.g. TPS54623


Those that have an input or programmable option for forced CCM (if for example you want to avoid the switching frequency changing, maybe to stay out of the audio band at certain times.) or give the option of a light load mode.


e.g. LM5160

When inductor current is positive, the top switch turns off first, followed by a dead time.  During the dead time if there's enough current to slew the switch node capacitance to ground (or a body-diode drop below ground), the lower switch will turn on with very little voltage across it.  Therefore zero voltage switching or soft switching.

If you are always in CCM and the inductor current goes negative, say at no load, then as the OP points out when the bottom switch turns off with negative current, the top switch soft switches, and if the FETs were the same part number they would both have about the same switching loss.  However, it's common to choose different FETs for the control and sync FETs to optimize conduction and switching losses depending on the conversion ratio, so there could be asymmetrical switching losses due to extra gate charge and transition losses on one FET vs. the other.