Why Do 12" Tonearms Cost So Much More Than 9"?

Cleeds, I just finished reading your link. It is a very lucid explanation of how skating force is generated. It also mentions both Reed and Schroder.
I miss understood what you meant by fixed horizontal bearing. Both the Reed 5T and Schroder LT do not have fixed horizontal bearings. The platforms that the horizontal bearings are mounted too are hinged at the right radius so that the pivot point of the bearing follows an arc as dictated by Thales thus the arms remain tangent to the groove at all points. This is shown by your author.
Your author relates the skating force should be 10 to 12% of VTF. 
Wally Tools says 9-11%. Some say the Wally Skater is the most accurate way of adjusting skating  https://www.wallyanalog.com/wallyskater
I have not tried one. It costs $260.00. It does however reliably gauge anti skate as a percentage of VTF. I use the Hi Fi News Analog Test Record and use the lowest velocity band to set anti skating. It coincides with Frank Schroder's method of using the run out groove area which Peter Ledermann also endorses. 
At any rate anti skating is a ball park measurement. There is no exact right figure and many opinions as which side of the ballpark you want to be in. I'm happy just to get in the ball park. 

Cleeds, before you suggest that I am silly I suggest you carefully review Schroder's patent so you know what you are talking about.

Cleeds, I am not sure what you are thinking, but just to be as anal as possible, I need to say that pivoting per se is not the cause of the skating force.  The skating force is first of all due to the friction between stylus and groove; I am sure we agree on that.  The friction is in a direction away from the stylus tip; the friction force wants to pull the stylus and cantilever away from the cartridge body and tonearm.  To resist that force, a counter-force is exerted by the cartridge/tonearm in an equal magnitude and opposite direction. The skating force is a vector generated as a result of that force resisting friction, because of the fixed headshell offset angle and also because of lack of tangency of the stylus to the groove. When a straight line can be drawn through the longitudinal axis of the headshell to the pivot point, and when the cartridge in that headshell is tangent to the groove walls, there is no skating force.  Among "conventional" pivoted tonearms, only underhung types, which all have zero headshell offset, ever meet that criterion in actual practice.  For such tonearms, that condition applies for the brief moment when the stylus is tangent to the groove.  For pivoted tonearms that work like the Schroeder LT or apparently the Reed 5T, that condition is constantly met across the surface of the LP, if they are perfectly set up. So, ideally (given diligence in set-up), those latter two tonearms would not generate a skating force.

Thanks for all the information on the Schroeder LT arm Mijo.
It looks and sounds like a near endgame proposition.
But.....$15 large?
Unless the right numbers come up on the lottery I doubt I will ever be giving one a test drive unfortunately.

I think they are somewhat less. I have a relationship abroad who say I can get one for $10,000 (wink wink). I'll have to pay VAT tax up front but I'll get it back in several months.
The dealer in the States is Xact Audio  https://www.xactaudio.com/analog/
He sells it with or without his own turntable but he modifies the arm replacing the wood for a magnesium arm tube which I am really not crazy about.  He has some kind of relationship with Schroder apparently he can sell them here in Magnesium and Frank can sell them in Europe with wood. As for his turntable there is too much mystery about it for me to be sure. I think if I could afford to today I would get a Dohmann Helix 2 and put an LT on it. That would certainly be a last stop turntable.
Not to worry Uberwaltz. This type of design will work it's way down market. Reed is working on a less expensive version now called the 5A.
It is another passive design energized by groove friction but it uses a dizzying series of arms and bearings to do the job. The Schroder is certainly a more elegant design. You have to love the magnetic track trick. One magnet + one bearing vs a bunch of arms and bearings.
The 5T is however a beautiful arm and boy does it work. It is $20K and very complicated but still a grand exercise of applied technology.