Largest source of inefficiency (and heat) in common isolated SMPS designs is output rectification, which has to run on frequencies ranging from tens kHz to units of MHz, thus the diodes there tend to spend relatively large proportion of operating time in various half-open-half-closed states. Active rectifiers somewhat improves overall efficiency for high output currents, but the problem with switching time still remains.

On the other hand for line-frequency rectifier times required for rectifier to switch can be ignored and only significant source of heat is voltage drop on rectification diodes (ie. 0.6 - 1.5V), so one watt of heat dissipation per one ampere of current is good ballpark.