Are large radiators supposed to be colder?
You don't think it needs bleeding - but have you tried? 99.9% of the time, that is the problem with water-type radiators and low heat delivery; lack of flow due to trapped air bubbles.

As a general rule, larger radiators do make more effective use of lower temperature water, but that does not seem to be your situation since you are evidently looking for more heat.

A noticeable stratification of the water temperature is not a good sign that things are circulating effectively, so I would draw the opposite conclusion that you have.