Rectifier, Flyback vs Transformer, Rectifier, Buck
The main idea of switching type mains supplies is getting rid of the huge, heavy, costly 50Hz transformers.

The size of a transformer is mainly given by its iron core, and that size is controlled by the amount of magnetic flux you have to run through it. You need the core "eat" all the flux (voltage-time) you are stuffing into it during one half-period of the AC on the primary side, before you demagnetize and counter-magnetize it again in the second half-period. The lower the frequency, the longer the time of a half-wave, the more flux.

If your core cannot "eat" all this flux because it's too small, the iron is overexcited and "vanishes" magnetically for the excess flux. You can see this as enormous peaks of primary current if you e.g. run a 120V primary transformer on 240V, or a tightly sized 60Hz transformer on 50Hz. The transformer overheats then, even if there is no secondary current drawn.