AFAIK their bankroll is not really that huge compared to the whale players that betting 10K and above on single bet.
Most of the active user here in the forum usually have a medium size bankroll and acquire wager by grinding it slowly on low house edge games like Plinko the. Combine it with slot games when they are participating the contest.
Patience is always the key if you want to accumulate huge wager and ofc luck!
Do wagering requires experience and discipline so that you can gradually wager to move nearer to wagering condition but at the same time you are able to control risk for making sure that wagering more won't mean taking more risk and losing more money.
So gamblers who are at top of leader board in wager contests have many things converge together. Big gambling capital to use, experience and understanding about wagering contest and tricks to save their initial capital as well as increasing their wager sum with time, then finding better chance of rank up to top of leader board.
It is not like an appropriate contest for newbies if they want to be winners and be safe.