Both McClain and Koch were members of Nasa's 2013 astronaut class, 50 per cent of which was made up of women, Nasa said.
McClain completed her first spacewalk on March 22 with NASA astronaut Nick Hague. Even with the changes to her body, though, she was uncomfortable performing the work in the larger suit.
Yet some users were supportive, citing the importance of safety.
It's been 35 years since cosmonaut Svetlana Savitskaya became the first woman to walk in space when she stepped outside the Salyut 7 space station. What would have been a giant leap for womankind will have to wait because of issues with women's spacesuit sizes.
NASA spokesperson Stephanie Schierholz elaborated in an email to Gizmodo.com's George Dvorsky.
Ms McClain, who Ms Koch was supposed to walk with, was working outside the station when she realised that a medium-sized upper half of her spacesuit fitted her better.
GREENFIELDBOYCE: So the available sizes are medium, large and extra-large. It would take hours of crew labor - not to mention some additional risk - to fix that in time for Friday.
On Twitter, Schierholz said "In this case, it's easier (and faster!) to change spacewalkers than reconfigure the spacesuit".
What exactly is the problem with the suits?This includes a Russian Progress, Northrop Grumman Cygnus and SpaceX Dragon that will bring the crew supplies for "various science investigations". Because of a lack of properly fitting and prepared spacesuits, McClain will not participate in the next spacewalk this upcoming Friday (March 29).
Which leads us to the latest development: astronaut Nick Hague (who wears a larger suit) will make the journey alongside Koch instead. Koch will become the 14th on March 29.
NASA has replaced Anne with Nick Hage to conduct their second mission on Friday, during which they will continue to install powerful lithium-ion batteries for one pair of the station's solar arrays, nearly identical to the first mission. McClain is now "tentatively scheduled" to perform her next one on 8 April.