That's an interesting question!
From what I understand the original congress, also called the "tobacco congress" was mainly a southern knife. They used them down south to cut tobacco plugs, etc. for inspection (?). I know the design was rarely ever seen north of kentucky. Even though many manufacturers in the north made them, they were mainly a market item for southerners. I believe it goes back to the mid 1850's (if Im not mistaken)?
Perhaps the smaller version was made as just an everyday carry knife? It makes sense to have a small one with a pen and a sheepfoot as you have more variety. I know most of the larger congresses had two sheepfoot blades as one would dull while being used (in the field) and they had another blade to take its place.
I think we have to remember that pocket knives were designed to be used. And certain designs, like the tobacco congress, were made to fulfill a certain purpose by design. Just like the original pen blades, they were meant to sharpen quills before the advent of fountain/ink pens. Or pruning blades were design to "prune". Spey blades meant for....well you know! etc. etc. etc.
Many blade designs have been retained by manufacturers over the years even though some of their original purposes are not much in demand anymore.