Pork vs chicken ramen
We don't know since we don't have the exact recipes, but I have a few good guesses.

Pork broth is made by boiling pork bones, so the thickness comes from the extracted collagen. Chicken broth is usually made by boiling chicken carcasses, which is mostly meat and bones. That will make a lighter broth since there's not a lot of fat.

I came across a recipe for creamy chicken paitan broth which is made with the whole chicken then blended and strained, so you end up with lots of protein in the broth which will make it heavy.

Another one uses chicken wings and that will extract a lot of fat, which will also make you feel full quickly.