Even a light dog can put a surprising amount of pressure through its feet if it wants to. Tiny paws mean it's all concentrated in a small area. My 10kg dog feels like he weighs a ton when he puts his front paws on me and leans into it.
I've no idea what it takes to trigger a mine, and I don't know if it would be enough, but a dog can press down harder than you'd expect.
I know what you mean, when they stand on you wrong it's surprisingly heavy. But it's point loaded. A dog doesn't do that while walking around on even-ish ground. The point is it would take unusual scenarios for a dog to get 100% of their weight on one foot, but any toddler does it every step.
I would think this works both ways though. Yes, smaller surface area is bad, but also reduces the chance the dog presses down on a trigger. But then again... they have four feet, but then they a narrower track, but then they probably take more steps. It's complicated I suppose!
I've no idea what it takes to trigger a mine, and I don't know if it would be enough, but a dog can press down harder than you'd expect.