Any fish should be able to hunt any fish of any size. You have to respect the intelligence of your players.
If the only way for a fish to grow is by eating smaller fish, of course your game is a ponzi, come on man think about it.
Are you in it for a quick cash grab, or are you really trying to make a tactical fun game ?..
If everyone can bite everyone, then big fish have no advantage, because a player with big fish risks big money, but does not receive any risk payment.
The fish grows not only due to bites, but also receives passive income from the actions of other players.