the gambling sites operate like any high end store, if you don't look like you're going to purchase after some time, you'll be asked to leave.
i don't even bother with it unless i've a minimum of 0.1btc (or alt equiv) to put down.
have had moderate to great success with this methodology.
I'm sure this is not true, most sites are provably fair so the size of a bet doesn't matter. You probably just tend to remember big bets better than small ones. Or maybe you go all in with tiny bankroll and quickly bust, while with bigger deposits you play more carefully. Anyway, this is some sort of confirmation bias, it's very unlikely that gambling sites are cheating.