Canoe weight is much the major factor for me due to the need to portage. It is not just the carry but that lift to the shoulders that is tough. I've weighed every canoe I took out.
As to what effects the weight I think length is a factor but consider the displacement factor. A sixteen foot canoe carries perhaps 500 lb comfortably. The fourteen foot canoe can carry the same thing but one will notice the shape of the hull is broader. The Old Town trapper is a case of a short canoe that carries a lot of weight due to its broad almost beaver tail shape. Bottom line is, it take a certain amount of wood to get a certain amount of displacement whether it is in length or width.
I have used the factory specs just as a guide and a relative guide at that. One thing is for sure, a given hull with Dacron is lighter than the same hull with canvas and filler.
Not to beat this subject to death, when I select a canoe to rebuild for the next year's journey I look at length, width, type of material, (Spruce vs Oak or Mahogany) shape of hull, thickness, spacing, and size of ribs, then seats. A lot can be gained by skimping on filler and paint and I guess that is the differences most often found in factory canoes.
I will admit, all other things being equal, for the same shape hull, finish and materials, a foot shorter would be lighter. How much? my guess, 5 to 8 lb. Someone can cut a foot out of a finished canoe and weigh it.
Does that help?