All it takes is a few seconds, depends on what they have the timing set at. The images I see can have many different factors involved. Lighting, angle of the screen etc.
Like I said I own quite a few of each. See them every day side by side. Use them side by side daily. Day in and day out.
And no that is not normal if the screens are really displaying those characteristics. They each have a IPS screen, same type of screen. One is brighter than the other due to it's size. The 27 has denser pixel density. There is no huge disparity between the quality of the screens other than the size. Seeing them in a store one time is a far cry from using both on a daily basis side by side.
I don't know what to tell you.