I think there needs to be a distinction made between
a limit to the size and weight of an aircraft that PPL can fly
and what aircraft a PPL can act as PIC. My understanding is that you may act as aircrew in a multi-crewed aircraft that is operated not for hire or reward with a PPL if the PIC is appropriately rated as a training captain. But if you want to act as PIC with a PPL there is a weight limit that applies.
That's my understanding of the rules, happy to be proven wrong.