Back..I wanted to look again as it struck me - which is really what you had said before - that the tightness is against the rib section rather than the stockinette. When I looked closely and enlarged the bottom pic you can see that your edging stitch is different in the rib than the stockinette and it may be this that is partly causing the issue.
What is the edge stitch on the rib section..as compared to the stockinette. I presume the latter is a knit. I thought both would be knit but they do look quite different (as may be apropos to rib style)
Again, I do think blocking will resolve quite a lot but it is bunching against the rib edge and I think trialing a little may help.
I would look at three options. Casting on with a half size needle up and work two rows and then resume recommended size. Or, perhaps changing the edge stitch against the rib section and/or adding fifth stitch to the bottom few rows or that rib section.
Have you enough yarn to make up a swatch and trial? The swatch would need both ribbing and stockinette section.
I hope one of the more experienced knitter will throw two cents in