Art,
True, but you must consider the relative power required to turn the larger tires. The larger the tire, the more power it takes to turn it at a given load.
For a given tire size; say 15x70, there can be considerable variation in their circumference. A place like Tire Rack will give you a comparison of revolutions per mile for various tires of the same size. Also, you need to consider the cost of replacement wheels and possibly hubs too. You can't just put 16" tires on 15" wheels, or 16" wheels on 15" hubs. You must also consider the clearance between the taller tire and the wheel wells, shock absorbers, and steering linkage - may be more trouble than it's worth.