Paku io is a great example of mass growing styled io games. You may find it similar to Piranh.io. When you reach next level your area resets and you can be beaten even by lower level player.
How to Play
The main aim of Paku io is getting to the TOP. The higher level you reach the higher is your position in the rating. You can get rating points by eating color dots and other players. And interaction with other players is a bit tricky in this game. There're two main characteristics: your level (dashed circle) and you current mass (filled circle). New level is reached when filled circle reaches dashed circle boundaries. But at level up the mass resets to its initial value. This means that at the moment right after level up any player can kill you because your mass is at its minimum. Also higher level means higher speed. It's almost impossible to get away from a bigger enemy.
The game is non-linear, by getting higher level you can't feel safe, you can be attacked by anyone at anytime. This makes gameplays quick and it's really hard to stay at the TOP even for a few minutes.