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-A certain amount of energy goes into deforming the rubber when it hits the road. The square footage of rubber that meets the road per rotation is a function of tire width and does not change with contact patch. In other words, a 195mm tire has 5.4% more area that must make and break contact with the road surface than a 185mm tire.
-In poor conditions, a fat tire means you displace more water and snow.
-A larger/fatter tire may mean more rotating mass, which can be a small factor in city mpg.
-A thinner tire will help shave those last few square inches off your frontal area.
-Fatter tires tend to have lower max psi ratings. Yes, a 225mm and 195mm tire will have similar contact patches at the same PSI, but chances are, the 195mm tire will allow for more pressure (and therefore a smaller contact patch) than the 225mm tire. I'm no tire engineer, but it would seem easier for a tire to preserve its composure under pressure when the distance between sidewalls is minimized.
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