IIRC the 49ers did a piece on him related to this after the signing.
Something to the extent of, "McKinnon scored highest on a set of advanced analytics related to physical strength and athleticism of any player available at the position". He can fly like a guy who is ~200lbs but can ground n pound or pass block like a ~260lber, which is what attracted Kyle Shanahan. He's like a super strong swiss army knife back, he can run it outside which is big in Kyle's system, he can also run it inside, hit the hitch, flat, or wheel and receive, pass block.
They also mentioned in the piece that at 5'9 he's always been a smaller back, even since high school, so he's always had that "because of my size,I have to work and train harder than everyone here" mentality, so he's trained like a mother ****er since early on.
Point being he is just extremely jacked, explosive, and unusually strong for a guy his size. He can play both bigger and smaller than his size when needed, which is why Shanny had such high stock on him.