Maybe he's carrying just a little of the bathroom wall around his middle
Seriously, he looks like he's at a pretty good weight. And, like someone else said, he'll probably drop some for the summer.
It's hard to tell with some dogs if you look at them from a vantage point of looking down on their backs. From there, Shiva looks like a butterball, but when you see her from the side, especially when she's moving, you can tell that she's not carrying any extra fat at all.