On Earth a spacesuit can weigh up 280 pounds on the ground, without the astronaut in it. But after last week's spacewalk, she decided that a large would be too big.
NASA said the switch-up has to do with what sizes of spacesuits are available on the ISS. "When you have the option of just switching the people, the mission becomes more important than a cool milestone", she said.
"In this case, it's easier (and faster!) to change space-walkers than reconfigure the spacesuit", she explained. Last week McClain posted a video of her doing a "loop scrub" on a suit, necessary to keep the cooling system running properly. It would have been the first time an all-female mission conducted the mission, which has usually been completed by an all-male or male-female team.
Earlier this month, the crew pointed out that suit sizes might be an issue, as McClain stated on her Twitter page that she had become 2 inches (5 centimetres) taller due to space growth.
Astronaut Nick Hague will make another spacewalk on Friday. "It really is the luck of the draw", Schierholz told Space.com.
"I can not contain my excitement!" she tweeted at the time.
"If that's a role that I can serve", she said, "it would be my honor to do that".
Nearly 35 years ago, in On Oct. 11, 1984, Sullivan took her first steps in space.
Both astronauts were members of NASA's 2013 astronaut class, half of which was comprised of women.
In 54 years of spacewalking, women have only gone outside with men.
The change of plans sparked a backlash from social media users, who questioned why the agency doesn't have enough spacesuits to fit the women on the space station. McClain had trained in both the medium and large sized units, but discovered that the medium size worked better for her.
The shuttle spacesuit was created to be made of many interchangeable parts, to accommodate the large number of astronauts with widely varying body sizes.
When asked whether girls can be astronauts, USA astronaut Anne McClain tweeted that while there are many obstacles on the journey to space, your gender isn't one of them. "Or, do we take that money and turn it towards the suit development for the next generation?"
Given the level of high diversity at NASA, Stephanie Schierholz expressed confidence that a historic all-female spacewalk will take place in the future.