Rob, 28, has withdrawn himself from the public eye completely, after putting on weight and suffering from depression and anxiety.
While his concerned family have discussed his problems on Keeping Up With The Kardashians, 34-year-old Kim thinks there's a simple explanation.
Speaking to Rolling Stone magazine, she said: "It's not the mysterious, what's happening with Rob.
"He has gained weight. He feels uncomfortable being on the show, and that's OK.
"Do I think he smokes weed, drinks beer, hangs out and plays video games with his friends all day long? Yes."
Something tells us Rob might not be too happy with Kim when he reads her comments.
Earlier this year it was reported that he was angry with his entire family after they discussed his life on the show.
A source told HollywoodLife.com: "He's angry that his personal challenges have been minimised to mere high ratings for the show."