Proportions and overall shape can really make or break how a livery looks on an aircraft. The battleship grey color scheme of United Airlines suited the Boeing 747 perfectly, its size and hump complementing the darker tones giving it a bold and almost intimidating look. On smaller aircraft like the A320 family and CRJs, it felt too heavy, and even on the 777 it lacked the same presence IMO.

On the other hand American Airlines’ polished metal livery worked best on single-deck planes like the Boeing 737, Boeing 757, and Boeing 767, where the cheatline aligned neatly with the cockpit windows. On the Boeing 747, the cockpit is raised so you can't really make it allign the same way you can with humpless aircraft.