If it matters.
I did not like the Corbin method of installing the securing screw with the head aft and the screws length adjusting the fore / aft position. While not exactly drop in easy, I obtained large OD washers and stacked them to obtain a more precise fore / aft position, AND the washers no have a large contact area on the rubber bumper when the backrest is upright. The Corbin design places all the load of leaning back or letting the backrest spring upright, onto the 1/4" end of the bolt and tries to pierce the seat pan.
As for large or small passenger and the room provided with a backrest. I agree that if the driver is in a reclined position there may as well be no passenger on there. Our setup has myself at 5'10" and wife of 5'6" and we are not huge people people but not small either having a pretty good fit.
I have explained to others if they spend a bit of time dialing in the Corbin backrest in both the Fore / Aft location based on how upright the post is, AND also take a proper Allen wrench and ride then adjust the backrest pad angle for best comfort the entire setup will work together and be more comfortable then just dropping it in and securing the screw. These things are stupid money to buy, quality is fair, but if you prefer a firmer seat and take the time to set it up, it works well.