Originally Posted by Slaninar

Thin tyres do provide slightly lower resistance, and lower weight. Fatter tyres roll better on imperfect pavement. They will hold your weight with lower pressure, so ride will be less bumpy, wheels will be more robust. The bumpier terain you ride, the faster are thicker tyres. 1.5 to 1.25 is minimal gain in weight and air resistance, but significant increase in air volume.