[22c]

That's simple, you just stake more coins. Staking rewards go back into the stake, so it is compounding (ie. you stake 100, get a reward of 1, you are now staking 101). As long as you don't unstake, your stake will grow over time.

If you want to stake more coins than you have, then you need to acquire coins by other means (ie. buy them off someone who doesn't want to stake them).

Stakers also have an incentive to trade some of their rewards instead of re-staking them, because they want the network to get utilised rather than simply be 100% staked.