The 1849 Pocket model was offered in .31 caliber, with either a 5 or 6 shot cylinder. In 1862 Colt did to it what they had done to make the 1860 Army. They took an 1851 Navy sized cylinder and made the front half larger in diameter. This so-called "rebated cylinder" made it possible to increase the chambers from .36 to .44 caliber, but retain the 1851 size frame with only a slight modification.
Returning to the Pocket Model; Doing the same thing allowed Colt to increase 5 chambers to .36 caliber. The 1862 Pocket Model of Navy Caliber and 1862 Police were the same size as the 1849 Pocket Model, but with a rebated cylinder in .36 caliber. The Colt and Uberti replicas are the correct size. The Pietta is built on the 1851 Navy/1860 Army frame, which is historically incorrect and the revolver is much larger then the original. Think of the Pietta as being an 1860 Army with a short barrel and 1/2 fluted cylinder.
Clear as mud?