Just a non-critical response, in order:
Being exposed to free air, the wires can be smaller, but these do look a lot smaller. The POCO doesn't adhere to the NEC, and this was probably an upgrade.
The (what appears to be) old range cable is a big hint about this being an upgrade. Are there any obviously-older cables? The pic isn't clear at the top.
Other than most likely not being approved for two conductors (cured with a simple pigtail), the neutral will not be overloaded, since the two hots share a breaker.
My guess is that the black wire was on the breaker above the shared one; we can only wonder why. If the breaker nuisance-trips, you could try moving it back.
I'd have a bigger issue with the pump cable, whether it's hard-wired or connected through an extension cord. It should be on a dedicated, properly-wired circuit.
Don't be suprised if you have a problem with the lower phase of the stabbed-on main breaker some day. One last observation: they have a lot of baseboard heaters!
