Ask the Editors: Why Are The Back Tires on Performance Quads So Little?
Excellent question and indeed, it can look a little disproportionate when compared to the higher profile of the front tires. Fortunately, there is a lot more to this than simply trying to look different.
The actual answer has all to do with handling; specifically, sport ATVs are typically rear-wheel-drive and are thus turned with a technique known as rear wheel handling. What this means is that turning the handlebars in the direction you wish to go is only half of the equation in making a turn. The majority of the turn is actually done when the rear wheels break loose and the quad performs a bit of a controlled drift.
When we say that the rear breaks loose, we mean that the tires literally spin past the point of traction and actually begin to slide atop the terrain rather than claw into it. The smaller diameter wheel and stiff sidewalled low profile tire makes this process far easier to accomplish than would a larger tire made of a softer compound (which would be more inclined to fold over when aggressively cornered).
Since utility ATVs and especially 4x4s handle quite a bit differently (namely they steer via input from the front end/ handlebars rather than flat-slide to get around corners), the wheel size is much more proportionate from front to back.
