This, again, varies by country and is complicated. As one example, according to
https://www.germancivilprocedure.com/disinheritance-lawsuits... (search for "Even lifetime gifts are relevant for the calculation of the Pflichtteil"):
> German succession law orders that all lifetime gifts made by the testator within a period of ten years prior to their death is added to the value of the estate. Such gifts are thus also basis for calculation of the forced share.
and a provision like that seems like a clear necessity to prevent people from working around the compulsory-share legislation by just giving their money away before dying. And yes, my understanding (which could be totally wrong!) is that this means that there's effectively a clawback provision on gifts in Germany.
So it's neither "fraction of the estate" nor "fraction of total wealth generated" but a compromise "fraction of what the estate would have been if not for certain actions on the part of the parents".
According to https://works.bepress.com/aaron_schwabach/3/download/ there are in fact such clawback provisions in various European countries (as of 2011, but I don't think that much has changed since then) with different details: time periods, whether you can claw back non-fungible goods from someone who purchased them in good faith from the gift recipient, whether children can officially sign away their rights to gifts at the time of the gift so they later cannot invoke the clawback provisions, etc, etc.
Yes, from a US-or-UK common-law point of view this all seems fairly counter-intuitive. ;)