I’ve seen several pine martens up close and they all seemed to be different sizes. Those that I’ve seen in trees (in WA and in Jasper) looked about the size of the a small house cat. But, once in the Washington cascades, at a place called Kennedy Hot Springs in the Glacier Peak Wilderness Area, I was in the hot pool and one came up and peered a me as though he was checking to see where the humans were–then he ran back towards my campsite. His behavior seemed suspicious, so I went back to camp to check on my belongings. Sure enough, what looked like a large pine marten was standing up and rummaging through my pack, which I’d left leaning against a log. It seemed to me the animal was more than the average 3 lbs., since he could reach into the top of my pack while standing with his hind feet flat on the ground. Since then, all the other martens I’ve seen seemed smaller than that individual. Reading now that the old-world pine martens were sometimes larger makes me wonder if there is sometime more variance in pine marten on this continent too.

Here’s one I saw in Jasper National Park in Alberta, Canada:

https://www.all-creatures.org/aw/pinemarten.html