Rousey was revealed last month as one of the cover stars, pictured stripping off and posing in a painted-on bathing suit.
"It's a real honour being part of such a historic issue that really pays homage to different body types of women and not promoting just one cookie cutter image for every woman, but showing that the healthiest version of every body type is the sexiest version out there," the 29-year-old said after the covers were unveiled in New York on Saturday.
"And I couldn't be happier ... I think I was just as honoured to get it as to be there in person to watch Ashley's reaction."
Graham, a size-16 lingerie model, is the second plus-sized star to feature in the mag's swimsuit edition - but the first to clinch a cover.
The 28-year-old is pictured wearing a skimpy purple and yellow string bikini as waves crash around her.
After the announcement, she burst into tears and said, "I'm shaking. I'm so honoured."
Meanwhile Clouson, 20, is a two-time SI swimsuit model who began modelling at 14.
"Hailey is the cool girl who doesn't even know how cool she really is," said editor MJ Day.
"She's not only every man's dream but also every woman's 'girl crush.' She's goofy. She's smart. And she's completely humble. What's crazy is you don't even feel like she knows that when you're around her."
Last year's cover featuring American model Hannah Davis left very little to the imagination.
Davis donned a microscopic bikini, even pulling down her bottoms to reveal her ... lower region.
Previous SI cover stars include Kate Upton, Irina Shayk, Chrissie Tiegen, Nina Agdal and Lily Aldridge.
- news.com.au
