Bill Browder explained it.

Putin’s 2003 arrest of the richest man in Russia, Michael Khodorkovsky (which included a televised trial with Khodorkovsky in a cage) prompted approximately twenty oligarchs in the country to plead with Putin, asking what they could do to avoid that fate, Browder explained.

“His response was: ‘50%'” Browder said. “Not 50% to the Russian government, not 50% to the presidential administration of Russia. 50% to Vladmir Putin. And at that moment in time, Putin effectively became the richest man in the world. I estimate his net worth to be $200 billion.”

https://finance.yahoo.com/news/fund-manager-explains-estimates-putins-net-worth-200-billion-105932448.html

Fund manager explains why he estimates Putin's net worth to be $200 billion

July 20, 2017