There are several brands that put the battery in a big diameter seatpost. The drawback is of course that this battery is proprietary.

For the fast spinning motor due to small wheels, its only valid for a hub motor, not for a mid drive motor because the mid drive motor output turns at the pedaling cadence which is independent of the wheel size.

Almost all mid drive motor are made of a fast spinning motor and a gearbox or internal belt that reduce the spinning speed to the pedaling cadence (the Pendix mid drive is an exception, its a direct drive mid motor but due to its low spinning its pretty heavy)..