Pietta makes the 44 Navies, and they are the same size as a 36 Navy except the frame is cut for the rebated 44 cylinder. You see a lot of them because they are cheaper.
The Uberti Navies are all 36 caliber, and properly sized. The 1851 Navy has a 7-1/2 inch octagonal barrel and a pivoted loading lever. The 1861 Navy, also 36 caliber, has the streamlined barrel and crawling loading lever like the 1860 Army, and has a 7-1/2 inch barrel and straight cylinder.
The 1860 Army shares the Navy frame size, but has the rebated (stepped) cylinder to enlarge the forward end to 44 caliber, and the frame water table is stepped to match. It has an 8 inch barrel and about a 1/4 inch longer grip-frame.
Long story short, The 51 Pietta 44 has the 1860 Army frame and cylinder, and the Navy's octagon barrel bored to 44 (45) caliber, and the throat shortened to match the longer Army cylinder.
The Ubertis are more authentic in size and shape, particularly the back curve of the grip frame.