In the vertical world the lean of the handle bar is what creates desirable spacing for a more compact carry bike rack. One of the biggest reasons we chose to have multiple size baskets was to control the lean and the angle of the handle bar. It allowed for a better and smaller design, creating a lighter rack, even lighter than name brand four carriers. That lean also controls the force direction or vector that is placed on the front wheel and fork.
The general size basket (one size fit all) create havoc between different width tire bikes, fatter tires stand upright more turning the handle bar almost horizontal to the ground and the skinnier tires lean much more with more of a vertical handle bar. This creates interference between bikes, causing rubs, and damage to the bikes and the rack. The solution, make it so wide that no matter what bike is in there it will be acceptable. Well, that gives us racks that are seven-feet-wide and weigh upwards of 100 pounds. Also, the lean of the front wheel when not correctly positioned and controlled with in a small range can put excessive force on the hub, rim, and fork (got high end carbon rims, TT bikes?), especially in high vibration cases on washboard dirt roads. Then add the different vectors (forces) on each bike and the rack can start to wobble and teeter totter. Another reason for controlling the lean with three different basket sizes. Keep shredding.