This is a straw man.
The real argument: someone with more money is less likely to become destitute as a result of a bad investment. Also: someone with more money is less likely to become a political problem that shuts down the market, or a drain on the public purse, when they lose money.
When it comes to private investments, someone investing e.g. $10k cannot afford to do legal diligence. They are also unlikely to unilaterally pursue someone who sued them in court. That almost guarantees they’ll be the sucker in the long run.