If all else is equal (i.e. same speed, same course, etc.), it will take more calories to carry more weight. But is that relationship linear? In other words, will a 225-pound rider (counting the bike) burn 1.5 times as many calories as a 150-pound rider?

It seems to me that there may be some other factors -- increased rolling resistance and increased frontal surface area come to mind -- that would make the differential even larger. But are those factors significant?