There are a couple of ways to determine the payouts of certain games. In the case of big software companies such as Microgaming, Playtech or NetEnt, the game developers are the ones who determine the payouts of the games they provide. There are some standard ways, used to define how much will a certain game pay. It is important that players still enjoy a fair outcome while the virtual casino also makes some profit in the long run.
However, there are also software companies that allow the web-based casinos to determine their own payouts. This can be a positive thing when two virtual casinos that use the same software provider compete in offering higher payouts. However, this can also lead to decreasing of payouts so that the online casinos can make higher profits.