I think he looks good. If they really wanted a bulky Flash, they'd just pad his suit and other costumes.
Honestly, I think a lot of the male actors playing superheroes go overboard. Would Starlord really spend hours and hours on the Milano in the gym bulking up? Would Wolverine really be that self-conscious about his appearance that he'd spent so much time bulking up since joining the X-Men. Superman has never made much sense, since he'd need special gym equipment for muscle building given his super strength (and muscular bulk is pretty much unnecessary for him). Batman does make a degree of sense, but the older gadget dependent Batman probably wouldn't rely too much on physical force.