as long as the breaker is rated so that Ib<In < Iz (of the smallest csa cable), it's fine.
Spurring off is ok obviously
Then you have fixed loads immersion etc where the CSA may be smaller than the fixed wiring and also accessories such as pendants where the wiring is smaller which is ok
If you take the cover off a DB and there's a 6 sq connected into a 20 amp MCB because someone has downrated the final circuit somewhere along it's length
Well I don't agree with that