Nah, the ulna flares a bit proximally, but nowhere near that much. I'm not at the lab so I can't snap a few more appropriate shots, this will have to suffice:
The tibia gets pretty narrow mediolaterally, that's not a remarkably small tibia.
Plus, you can pretty clearly see substantial rugosity (bumpiness) of the bone where I've indicated with an arrow - this type of bony growth only happens when a powerful muscle repeatedly pulls on the origin/anchoring bone. There's nothing that powerful that originates that distally on the ulna, but that's right where tibialis anterior originates on the tibia. Tibialis anterior is the shin splints muscle - it's more than powerful enough to cause the observed roughening.
NSFW http://i.imgur.com/kyBvC.jpg