My gut feeling is temperature and constant deep discharging probably explain most of the issues with Leafs. Like a lot of things there is a curve with a knee and it's easy to drive a leaf so you're on the wrong side of it.

I don't really know how Leafs batteries are connected but one could also expect that a smaller battery pack has less redundancy. If due to bad luck a couple of cells go bad you lose a lot of capacity. More likely to happen the less redundancy you have.