It's intuitive. 1st generation, 3 children. They each marry and have 3 kids each. Those are now 15 people with a direct "right" to the wealth. Third generation

It's intuitive. 1st generation, 3 children. They each marry and have 3 kids each. Those are now 15 people with a direct "right" to the wealth. Third generation all marry and have 3 kids each as well, now you have 51 people with a direct claim to the wealth. The family has grown from 2-51 in one lifetime, that's a 25x growth rate. But the trust money is probably in safe, boring investments that grow at modest rates, so the growth rate of the family simply outpaces that of the money over time to the point where it becomes trivial for the family members to piss it away