Most of your older refrigerators were frequently over powered for the job at hand, so their cooling power is very good. They ran in quick, short bursts compared to newer units. Some were insulated quite well too. Despite the powerful cooling systems, many are rather efficient, especially the older non-frost-free type. This is because they don't have heating elements in them that add heat to the food compartments while defrosting. They also don't have fans in them which consume energy in their own right.
Leaking cooling units are generally something that won't be an issue on an old refrigerator. Unlike an automobile's air conditioner, there's really no reason why the coolant would leak out...it's a sealed system. This is one of the reason why old refrigerators from the fifties are still rather plentiful. I had an old 1948 Fridigaire fridge that I sold on E-bay last year, and it was still going strong when I got rid of it!
Todays units that are designed to consume very little energy, and are highly insulated. Newer refrigerators absorb very little energy, so the refrigeration mechanicals are smaller, and run longer. Part of this too is because the electronic thermostats nowadays are also more accurate and can keep the temperature from fluctuating as much. One of the drawbacks to this however is that they have a hard time coping with major temperature loads, like if a lot of thermal mass is suddenly added to the fridge....bring home several huge cases of pop and put it inside your new fridge and notice that the compressor will run for HOURS trying to bring it all down! Same goes for when you turn the fridge on from where it was sitting off for any significant length of time.
The worst era for refrigerators, as I see it is the late sixties through the early eighties. Despite the fuel crises that occured in the seventies, energy consumption guidelines did not really kick in till about the mid 80's. Before that time, refrigerators had very thin walls, as manufacturers tried to expand the interior space versus exterior space consumption. Manufacturers went to bottom-mounted condensors that radiated heat energy back into the food compartments, and they generously used fans and large cooling units to circulate air and compensate for all the heat energy that was leaking back inside. That's not to say these machines were not competent at keeping food cool and fresh, but they consumed a LOT of energy in the process!!!