There are 2 issues here. If you are riding mountain bike tires with treads, definitely change them out for road slicks. You'll see an amazing difference.

The other issue is rolling resistance and honestly I would't worry about it if you are biking for transportation. You are not going to notice that much difference. This only will matter when racing where seconds count.

In general, small diameter wheels are going to roll faster, but there is some evidence that there is a limit to this. 23mm wheels can actually have less rolling resistance than 19 or 20 mm wheels in real world conditions (i.e. not on the track.) Also, latex tubes have lower rolling resistance than butyl (but are not as puncture resistant.) Otherwise, you are looking at differences in how the tire is manufactured. Different tires from the same manufacturer in the same size can have wildly different rolling resistance. More on this here for those who are interested: http://analyticcycling.com/ForcesTires_Page.html