Maybe it's simply because Ukrainians aren't killing Russians within territory they control, so they don't have as many bodies to exchange. Looking at body exchanges to determine the number of dead people on either side seems just...like a weird metric?

It’s a reasonable metric but it should be evaluated based on what’s happening on the ground. For example, if Russia is advancing and Ukraine is retreating, and KIA is same in each side, then Russia would pick up many more bodies to exchange than Ukraine.