Julia wins, obviously. Being tall doesn't give you any strength and stamina, but bodybuilding and having arms, legs, a stomach and a back from hell do. Ekaterina's both arms together are thinner than one of Julia's, and Julia's are all muscle. I did a poll with Ekaterina fighting Katy Perry and Ekaterina won, but I'd ascribe that to her being a professional athlete in addition to her height. Think of Jessica Pardoe who is 6'10" and has lost every single fight so far because she's as thin as Ekaterina but spends her time apparently getting drunk instead of doing sports!
I also picked Julia to win, though I do think you're perhaps dismissing Ekaterina too easily. As you mention, she is a professional athlete, and as a volleyball player I'd imagine her stamina is actually quite good. Likewise, the strength training involved in powerlifting focuses far less on stamina than strength. Ekaterina does look to have some leg strength, though obviously not in the league of Julia.
My read on that fight is that if Julia can keep a hold on her foe, she could easily inflict enough damage to prevail. Ekaterina's best strategy would be to use her considerable reach advantage coupled with movement to wear the bodybuilder down. If she could get Julia gassed, she might then have a chance with, say, a figure-four head scissors or such.
In all, Julia would be the way I'd bet, but I don't see the result as a given.
-JM
