Turning to the book , "A Trappers Guide" , Samuel Newhouse (1865), in his first chapter he notes:
2. The pan should not be too large. A large pan, filling nearly the whole space of the open jaws, may seem to increase the chances of an animal's being caught, by giving him more surface to tread upon in springing the trap. But there is a mistake in this. When an animals springs a trap by treading on the outer part of a large pan, his foot is near the jaw, and instead of being caught, is liable to be thrown out by the stroke of the jaw.-- Whereas, when he treads on a small pan, his foot in nearly in the centre of the sweep of the jaws, and he is very sure to be seized far enough up on the leg to be well secured.
Some trappers would argue here. I would agree with Newhouse, except that now we are entering a change in traps. Before Newhouse and Oneida, traps were animal specific, meaning that a trap was built for an intended animal. This change to a smaller pan is seen about the same time that more traps became more universal, one trap is now good for several animals. A small pan would be good for some animals, but a poor change for others. Steel traps had largely been used for beaver, and the large pan worked well for many years on these older traps. Could it be that Newhouse was pushing the small pan just to same metal? I don't know if I would go that far, Newhouse was no slouch at trapping, and he made the finest trap in my opinion, possibly he was just stubborn and set in his ways.
Pan size has gone back and forth some over the course of history. Today many trappers use an after market pan extension, or modify the existing pan to a larger one , I'm a fan of those guys! However, most traps went to this small round pan and it's remained the standard for well over a century now.
Newhouse set the pace for other trap makers, if he is the one to change to this smaller size, it's likely that all others followed suit, and it probably showed up in the company piggy bank as they followed along.
An older tranverse pan trap, a pan made to fill the entire area inside the jaws.
A double leaf spring, the pan fits the jaws, it's a bit more narrow on the sides to clear the springs.
Montreal maker "Isaac Veal" , a single leaf spring with a pan large enough to fill the jaw area. Just enough room to clear the spring.
From the Museum of the Fur Trade, a Watkins , this is the average pan size of a common beaver trap. These measure about 3 1/2 to 4 inches square.
Three Newhouse traps, #2, 3 and the top is a #4. The round pan is just under 2 inches.
I might add that most traps that we find today all have the same size round pan. Victors and Triumph will have slightly different styles, but will interchange for most years. The number 2 traps will fit the #4's. Bridgers, Sleepy Creek, and Duke traps have differences, but remain about the same size. If you are a die hard fan of large pans, it's not difficult at all to modify these.