This is their business marketing strategy that they will always show you big wins but that doesn't mean they are fake and if you take it too seriously and bet aggressively in hope of earning lot of profits same to that then you can loose the amount which is wrong.
You only know you can win big with high multiplier or not if you practice betting with high risk and high chance to win big. Although, big wins are not common, a more important point is if you win either small or big and want to submit a withdrawal request, a legit casino will approve your request and process it for you.
Rollbit has good reputation and I am not doubtful about their legitimacy at all.
So always bet in your limits and if you are lucky small bets of $1 or even lower can turn into big amount with high multipliers.So don't get away with these things and have self control in gambling which will keep you alive financially.
At the end of a day, your net result, win or lose is more important than whether within that day you have a big win or big loss. Because after all, will you go to bankruptcy or maintain your initial capital as well as have a minor or good increase of it.