I will try to clarify my question.
As far as I understood, the proof of stake relies on the fact that it would make no sense to try to steal/forge transactions because you would lose your staked money.
However, what if the transaction you are verifying is transferring far more than you had staked? For example, I had staked 50ETH, but am asked to confirm a transaction containing 1000ETH. What is the incentive to not mess/steal those 1000ETH, if I only ever stand to lose those 50ETH I staked?
I just need a layman explanation, not detailed math.