Lighter is better, but smaller diameter tires will sink deeper in potholes and move at steeper angles. Plus spinning faster.
Also, some of the bigger tires have less rolling resistance than smaller. Michelin and Nokian both told me 14 had lower RR than their 13 size in the same tire model.
Check out Loremo.
Intro*-*Loremo - low resistance mobile
These are my new wheels. Very light, and only $50 for all 4.
I am using Nokian H silica compound tires. Not only low RR, but also very light, 2 or 3 pounds less than average weight. Nokian H won first place in German tire test this year.
Nokian H wins the German ACE tyre test