The way I heerd it, is that the distances between water tanks on the Ouray branch was too far apart for a 2500 gallon tender. Seems they lost one in the late 30s somewhere along the way and the spacing was goofed up. Anyway, whenever a C- was assigned to the branch in the 40s it would get the tender with the larger cistern so that the loco could make it between tanks.
When the 340, which had been in standby service to the 318 at Montrose, was deemed to be surplus the big tank stayed with the 318 and the little 318 tank went to Knotts Berry Farm. It has the really funky F&CC trucks. Up until a couple of years ago (1991 when the GT&C "40" became the D&RGW 340 again!) you could still make out the large 318 under the layers of paint on the rear of the tank. They took it down to bare metal for the rebuild.