You are not considering the angle of coverage. This differs from lens design to lens design. While I don't know of a 210 mm that will cover 12X20, there may be one that I have missed.

The short lens that I have heard will cover is a 355 G Claron. I use the 450 mm Nikkor M on my 12X20. This focal length on 12X20 seems to give me almost the same angle of view as my 210 Dagor on 8X10. I do know that my 210 Dagor will definitely not cover 12X20.