If you paint for 30 minutes then the compressor will run the same amount of time whether has a 60 gallon tank or an 80 gallon or a 20 gallon for that matter.
That is correct, it will run the same amount of time. CFM is CFM no matter where it comes from. If I had a 20 gallon tank the compressor would run more often, but shorter times. On my 80, it runs longer, just not as often. From the way I time my coats, the compressor runs once per approx 30 minutes. My air temp after the piping is just a few degrees above ambient with the compressor off and tends to jump about 15 degrees with the compressor running, and cools off pretty quick. I have forced air ventilation around the compressor helping it stay cool.
I understand the theory that pumping X amount of air creates Y amount of heat, whether that is done at once or over a period of time. My gun takes 13cfm (15cfm compressor) and I can get an entire coat on a full size pickup before it kicks back on. Add in flash time between coats, and the compressor sits idle for a good long while. If the compressor kicked on 4 times during the coat, I could not keep the air nearly as cool. 15 degrees isn't much of a difference, but in high humidity, there is quite a difference on how fast my desiccant dryer is used up. When the compressor runs continuously for long periods, I can get a 50 degree jump.
I also specifically stated in my first post that 60-80 gallon is negligible and any point you make in saying otherwise is utterly ridiculous. No nowhere did I state that "I didn't agree with the blanket statement that the 60 vs 80 gallon didn't make a difference" as you said. HR has a hive-mind on this subject, and while being generally correct, is just that, a general statement.
I also originally said: "If I had a 20 gallon tank while painting, I would see the same issue as I do now with the DA." My gun is 13 cfm at 26psi, compressor 15 cfm. A 20 gallon tank would empty very rapidly causing the compressor to run near constantly while spraying. I know that with my DA, which causes the compressor to run near constantly water is an issue. I could not effectively dissipate the heat from the compressor on the fly in 95 degree weather, such as today, when the compressor rarely shuts off. I can get a full coat, or nearly a full coat without the compressor kicking on with my 80 gallon. Although the air heats up when the compressor kicks on, it has plenty of time to cool back down between coats.
This isn't just for painting, but for any non-continuous use tool, impacts, air hammers, etc. There may be a time when you will impact for 20 minutes straight, but that is far and few between. There are times when I need all the cfm I can get, the blaster and DA are my only ones, but for 80% of my air usage it is intermittent.