[Pancho]

Is the 51 Navy physically smaller than the 1860 Colt Army? If so, is the 1861 smaller too or is it the same size as the Army just chambered for 44 cal. round?

[dwave]

Well, both are .36 Caliber and the handle and frame are the same. According to EMF, their replica (Pietta) of the 1861 has an 8 inch barrel and a creeping loading lever. The 1851 doesn't have the creeping loading lever and the barrel is 7 1/2 inches. The 1860 Army is almost the same as the '61 but in .44 and a bigger handle.

[Roswell 1847]

The 1860 Army actually has the same frame size as the 1851 and 1861 Navy pistols. The frame is different only in the relief cuts for the rebated cylinder and shoulder stock cuts.
As mentioned the Army grip is longer.

The 61 feels a hair lighter to me but that may just due to a shift in balance point.
Any actual difference in weight would be minimal.

[Old Fuff]

The only difference between a true 1861 Navy and the older '51 model is the barrel and rammer assembly. The '61 barrel is styled after the round 1860 Army, but 7 1/2" long (like the '51 Navy).

Some reproductions are now being made that use the 8" 1860 length barrel, but bored for .36 revolvers.

Besides what's described above, the 1860 used a modified Navy frame that could hold a .44 cylinder, and had a slightly longer grip. Otherwise the 1851 Navy, 1861 Navy and 1860 Army use the same parts.

[ArmedBear]

The Pietta 1861 replica is not accurate. Uberti is.