That really depends on the game, but with most online games you won't have to play the maximum bet. However, there are a few games that have a higher chance of you winning the jackpot if you play with a higher stake.
There are also some games that adjust the jackpot size in proportion to the bet size. You can find information of how the jackpot is won in the game rules of the jackpot game. One thing to keep in mind is that these are high risk games, and you shouldn't play it if you can't afford to lose the money.