Experimental efficiency curve of a buck converter
If your DC/DC doesn't have a synchronous rectifier (i.e. it uses a diode), then your analysis is generally correct.

The major losses in the DC/DC are:
1) Diode -- inversely proportional to duty cycle (VOUT)
2) FET RDSON -- high side switch -- proportional to duty cycle (and load)
2.5) inductor I^R and core losses
3) Switching losses -- proportional to frequency -- constant in your case
4) bias currents etc -- constant

If you keel RLOAD constant, then the FET RDSON losses will increase with VOUT significantly -- as you increase duty cycle, output current will increase, and so will the % time this is flowing in the HS FET. FET losses will increase as duty cycle ^3.