Wheel diameter is somewhat standardized at 0.75" (edit) for majority of applications. But the larger diameter wheels roll over an obstruction easier, such as an aggressive lobe. Again with the higher end lifters suited to bigger cams.
A narrow wheel reduces friction (less contact patch/square inches)...but you need a certain amount of width in order to prevent too much load in one narrow spot on the lobe (think of a cutting tool on a lathe). Additionally you need enough wheel mass/material for stability and so you don't crush the wheel...fun balancing act for the engineers.
For your last question - more balancing act stuff-you want the smallest, lightest, strongest wheel that satisfies majority of the applications. Once someone gets to the level of smashing the $400 lifters, they go to the $700 lifters...then the $1200, and so on.
