Here's a very good explanation from someone who knows (and can explain) HVAC issues better than anyone I know: "This is pretty basic. You can't take a 2" water line, reduce it to 1/4" and then reconnect it to a 2" line and expect great water flow. Supply air and return air need to be properly sized. If the return air is undersized you're starving the unit for air. Numerous nasty side effects occur when you start to restrict air flow. In the heating cycle the lack of air flow could be causing the high limit to start turning the burners on and off as there is not enough air flow to carry the heat away from the heat exchanger. Obviously this isn't a desirable condition as one of the side effects is adding a lot of stress to the heat exchange (overheating and cooling down repeating itself numerous times during a heating cycle) which it wasn't designed for. Efficiencies of the furnace tank, outlying rooms, farthest from the furnace, do not warm properly etc. What you show in your picture is cause for concern. You can see how much the R/A has been reduced from the size of the R/A opening on the furnace itself. If the manufacturer wanted a 12x12 duct attached to the furnace they wouldn't provide a 24x24 opening. If you knew the CFM of the unit it would be easy to calculate proper return air using a simple Trane Ductulator however, for the purpose of home inspector discussion, flag it as a possible defect and recommend (insert boilerplate here)".