The evolution of smaller arms in theropod dinosaurs
By Kevin Welch
Carnivorous theropod dinosaurs may have had brilliant breast displays that were better shown off with less distracting arm size. At mating time female theropods selected males with smaller arms, for it was in the display that male potency was better measured. Arms blocked and confused the message.
Today many dull, dark, and drab colored birds have complex breast displays with bright colors, bold spots, streaks, and bands. Birds evolved arms that were held at their sides to maximize their displays since they had to maintain certain wing size for flight. Large theropod dinosaurs didn't have such needs, so arms simply aimed at getting less obvious by getting smaller.
Carnivorous theropods that hunted live prey would also do well to reduce ostentation to the breast where it could be directed or hidden at will. Loud fluorescent patterns and colors wouldn't much help a large stalking predator.
Breast displays could be accomplished via skin pigmentation or short feathers. Feathers would be additionally useful by providing iridescence. Feathers could be made erect; and with head shaking, strengthen the display. It would also be a protecting layer where the body made contact with the ground.