I have never won big too,, but if someone withdraws big money (let's say withdraw 3 BTC in a day) from an online casino he/she wins is that considered normal? for large withdrawal cases, I heard that some gambling sites implement KYC, to prevent crimes such as money laundering or fraud and even maintain the gambling site's financial balance.
I often play in casinos and do not believe that it is possible to win large sums. It seems to me that only small winnings are possible in the casino. Even if you play a lot, it is almost impossible to win one, but a very large prize. You have to be very lucky to win a really large sum.