A low resting heart rate is a relatively good guide to general cardiovascular fitness, however during exercise it is more a factor of the % of your max HR. and everyone's maximum HR is different. The 220 - age calculation is of little real value, few people actually conform to that, it is mearly a statistical average with a wide margin of error. To find what your true MAX HR is you would need to do a stress test, but this should be done under doctor supervision. A person who is more fit will be able to do the same work load at a lower percentage of their maximum heart rate whatever that may be.
In reality a truer measure of your fitness would be at what % of your lactic threshold you are working at but that is a bit more difficult to determine and will change as you get fitter. Your max HR howerver stays relatively constant, it does decline slowly with age, roughly 1 BPM per year or less. For me it has been less than 1 BPM per year, I haven't tried to determine my MAX HR for a few year now but the last time I did, mine was at 190, and that was when I was about 45. Blows the 220-age right out of the water.
As to the higher calorie burn, he did have to move an extra 20 kg of mass that you didn't, so it is not surprising he burned more calories. Next time you carry a 20 kg back pack and see what results you get, I'll bet you will burn more than him then and be more out of breath too.
That's why some fat people are fitter than skinny people, they have to carry around all that extra weight 24/7. They already have some extra muscle mass hidden under the extra fat just to carry that extra fat in the first place.
You can be fat and fit, but you can also be skinny and unfit. Fat and fit are not mutually exclusive.
