Gambling site is not your private wallet so why will you be worried about having large amount in the platform? Unless you want to use same to play more games, you can withdraw to your private wallet, leaving only the amount you plan to continue gambling with. There have not been any bad record from either of the platforms, so I don't see why heavy gamblers cannot use them.

Most big gamblers prefer remaining private, that is why you will never know that some people are betting and withdrawing amounts bigger than those you mentioned. One clue you should know is that both platforms have been around for a while with no bad reputation. Rollbit is even expanding and have launched additional platform (Duel Arena) which should allay your fears.