The smaller the refrigeration system the less you will generally spend on the reclaim system also. True it may not be worth trying to reclaim the heat from a self contained household refrigerator - we do reclaim that heat anyway if the refrigerator is located indoors. It cost more in building cooling costs if it is indoors, but during heating season it simply lessens the heating load.It is the arrangement of ducting, insulation etc., that may cost more than the return from the heat recovery from small size refrigeration system. I base this from heat recovery scheme for exhaust gas from diesel generators, which is an economical proposal for gen sets of size 500 KW and above.
For a more permanently installed refrigeration system like you may find in a supermarket - even a small mom and pop store, there is significant enough heat to be reclaimed it is worth the investment.
I find it very hard to believe there is not enough heat in the exhaust of a 500KW generator that there is no way to extract even a little bit of it that is not worthwhile. Now if the generator is for standby only and that heat source is rarely available - that is a different story.
This is pretty much what they did with gas furnaces when they made them 80-90-95% efficient. They found ways to keep more of the energy in the heat exchanger instead of letting it escape out the exhaust - makes a lot of sense when the primary purpose of the appliance is to put out heat.