Very small transformer powering a microwave oven
If you double-check your wiring diagram you will find that is NOT a transformer. That is a common mode choke to keep RFI from being transmitted out through the mains power cord.

There is no way (with currently-known technology) to make a mains-frequency transformer that small that handles as much current as a microwave oven draws. You have seen how big the transformer needs to be by your comparison with the size of the high-voltage transformer.

Ref: https://en.wikipedia.org/wiki/Choke_(electronics)#Common-mode_chokes