This is confusing!
Given that some report an increase in MPH from a smaller tire, what is the reason?
Jonny Hotnuts comment about X-section is correct, and the John Player Norton demonstrated that a plug of still air created by the front wheel can be used to aero the front of the chassis.
Intuitively, it seems to me that a larger radius (tire section) would be an improvement in drag as to how it would disturb the air?
Is this less important than the extra rolling resistance of the contact patch?
If yes, that suggests that a really large diameter tire (= smaller "dent" in the salt) might be better.
Empirical experience has been that a tall thin tire with the proportions of a Frisbie has the least drag (but putting Moon disks on it still helps).