In the olden days, every room had a supply and return. That is best, but costs a lot. Yes, badly sized ducts and duct leaks cause pressurization or depressurization and turn your air handler into a power vent forcing out conditioned air, and maybe worse, sucking in humid outdoor air where water condenses at the first cool surface it hits. Even with good, well-designed ducts if you have central return(s) closing and opening doors can cause pressure differentials if proper room to room flow is not designed in. A home should be every so slightly pressurized.Main HVAC unit in my house finally pushed me too far last year and I had it replaced. It was a 'nice' unit at the time it was bought, scroll compressor, high seer rating.
As part of the replacement, my HVAC guy went through and did airflow measuring/rebalancing at each air vent along with the return line itself.
Turns out, the return line was only sized for about 60% of what it should be. This creates negative pressure on the return, which pulls air into the unit from wherever it can... and this is almost always UNconditioned air.
Both old/new units were 4 ton. The old ran nonstop on days above 95.
The new one still cycles off/on like normal on 95 days, and the compressor never gets out of the first speed.
So, boys and girls, airflow matters. Duct sizing matters. Air leaks, especially HOT attic air, really matter. House has never been this comfortable either. Indoor humidity is lower also.