Expanding isn't "reacting to heat".
What? What do you call it, then? I didn't say it was reacting with heat. Although it does react with oxygen in the presence of heat, to form NOx, that's explicitly not what I was talking about.
But, back to the original point: Diesel engines take in much more atmospheric air than gasoline engines when running at normal loads (highway cruising).
Yes. But they consume no more when wide open. The size of the exhaust is defined primarily by the maximum flow, not the cruising flow. (That defines other design characteristics more.) Diesels tend to have higher peak boost in spite of their typically higher static compression ratios, but they also tend to have significantly lower RPM limits and tend to run less RPMs while cruising.
Of course, all of this has been muddied by the introduction of the direct-injected gasoline engine, and by developments in diesel engine technology. Not only do GDI motors have higher cylinder pressures and thus higher temperatures, but there are also now diesels with [automated] throttles. As well, the recent crop of automatic transmissions with many gear ratios (8 now being common, 9 not being uncommon, and 10 beginning to roll out) and multiple overdrive ratios has led to gasoline engines being used at much lower RPMs...