Backpacker is right.
There is not (so far as I am aware) any kind of PPL weight restriction, HOWEVER the SEP (Land) class rating only allows you to fly aircraft up to 5,700kg with a single piston engine - with the notable exception of the piston engined PA46 Piper Malibu which for some reason best known to the CAA requires a type rating if it's on the 'G' register.
Interestingly an FAA licence works a little differently. Instead of SEP (Land) you have an Airplane Single Engine Land class rating with which you could fly the very same Malibu (if it was on the 'N' register) - even a turbine engined Cessna Caravan!
As someone would undoubtedly have pointed out, it's actually a little more complicated than that. For an FAA PPL holder to fly a Malibu they would need to have received training (and had their logbooks endorsed) in respect of pressurised aircraft, retractable gear etc.