I can't comment for DL or NW but I know that in the summer CO were blocking off 25 seats on TXL-EWR, I've never seen the route carrying any cargo Westbound either obviously due to payload restrictions.
I wouldn't be surprised to see payload restrictions on EDI-ATL, even if DL's B752s do have a longer range, they also have more seats compared to CO (184 compared to 175), so the difference in range is likely to be negligible I'd of thought.