Io is the chief blacksmith of Themyscera, it would only stand to reason that she would be of muscular size.
Chief Blacksmith of Themyscira, Io has been working in the forge for so long Diana was able to recognize her by smell alone. After Paradise Island was restored after its destruction during the Imperiex War. Themyscira was divinely blessed as a weapon-free zone and as such, all weapons Io made would instantly vanish on completion, much to her annoyance. Ares however once visited her while she worked, complimenting her latest blade before leaving, this blade however remained. Later on Io would be one of the minders of Circe's and Ares' daughter; Lyta. And would be the unfortunate to be alone with her when Ares returned to reclaim his daughter, Io managed to wound the God of War before she was struck down and Lyta taken away, Io would later recover.
Io would also have the bad fortune of later on being the one Amazon Zoom (Hunter) picked out of the crowd of Amazons when he attempted to make Diana a better hero, in Io's case by punching her two hundred times in less than a second.
Post-Crisis Io has been accredited as the inventor of the Purple Healing Ray and its weaponized counter-part; the Purple Death Ray, though she prefers not to be reminded of the Death Ray.
Wonder Woman - Vol.2 (1987) Edit
Issue #196 - Down to Earth, Part One (November 2003)