Originally Posted by alcjphil

The thing is that the smaller the cog, the less round it is. Cogs smaller than 13 teeth force the chain into a very tight turning path and don't engage the teeth of the cog as smoothly. Also, as noted earlier in this thread, small cogs wear much faster, 11 tooth cogs have fewer teeth engaging the chain than a 13 tooth small gog. This wears the chain faster as well. All this wear is further accelerated by a mid drive motor assist
The issue has nothing to do with the derailleur