"Pumping iron was the decision-making factor," Arnold said, adding that Pratt asked to workout with the former California governor when they first met.
"He's a strong guy, there's no two ways about it, and I'm very proud of him," he continued. "When I watched him make these moves in the gym, you know, the incline press specifically did it. Why I wanted him to become my son-in-law: It was the incline press."
The legendary action star said he was "blown away" when Katherine, 29, said she was going out with Pratt, 40.
"I said to myself, 'Do you have to be so competitive?'" he said sarcastically. "Do you have to have a guy that is taller than me, that is bigger than me, that's doing bigger movies than me and all of this kind of stuff, that makes more money than me?'"
On a serious note, Arnold said he's "really happy" about the new addition to the family.
"[Pratt] is such a great guy," Arnold said. "Not only a very talented guy and a great actor and a great star and all this stuff but a really kind man and kind to my daughter, which is the most important thing for me."
When asked by Kimmel if Pratt asked permission to take his daughter's hand in marriage, Arnold replied, "Absolutely, yeah. Very traditional way, yeah. Very good."
Pratt popped the question back in January after a year of dating Katherine. The couple were married in June.
The wedding marked Katherine's first marriage and Pratt's second. The 35-year-old actor was previously married to Anna Faris, with whom they share a son, Jack, 6.