When you say "smaller wheel" are you referring to differing effective rim diameters at the same bead seat diameter? (e.g. An aero 700c rim will have a smaller effective rim diameter than a typical low profile 700c rim)

If so, then this is something different than what I was referring to which is comparing different tire diameters (As a starting point consider 32" x 2.00" vs 20" x 2.00" tire. If both are on the same internal width rim and have 100 aspect ratio the 32" tire would have 67%* greater tire volume. Therefore in order for the 20" diameter tire to have equivalent tire volume as the 32" x 2.00" it would have to be much wider than 2.00". The math works out that the larger diameter tire has a smaller frontal area when it's tire volume is the same as a smaller diameter tire. This is why larger diameter tires at the same tire volume as a smaller diameter tire have better aerodynamics.)

*67% rather than 60% because the calculation is done with the average of the inside and outside tire diameter rather than just the outside tire diameter. Therefore the calculation is 30/18 for 67% rather than 32/20 for 60%.