Costume designer Mayes Rubeo recently revealed how the Tony Stark jokes came to be. In the film, Bruce Banner is forced to wear some of Tony's clothes when he finally regains control of his body. But Tony's fashion and style doesn't line up with Bruce's.

Whenever he is not Hulked out, Bruce is usually wearing a lab coat, engrossed in an experiment of some kind. The MCU’s Bruce sometimes drops the coat in favour of neat-casual attire in the lab, though he wears one briefly during Age Of Ultron.

Ever since the Hulk first appeared in The Incredible Hulk #1 (1962), comic book fans have written to Marvel asking Hulk co-creator Stan Lee how the Hulk keeps his pants on when he changes form. Lee actually offers an explanation in one interview, stating Reed Richards of the Fantastic Four outfitted Banner with some “elastic trousers” (possibly made of the same unstable molecule fabric the Fantastic Four’s costumes are made of) that stretch when he transforms. In later interviews, however, Lee admits that, despite his desire to provide scientific explanations for all his superhero’s powers, he still couldn’t understand how the Hulk kept his pants on.