Its basic probability. If you have a system that gives a winner 70% of the time, the only way to erode that probability is to run very few trades. So if you only ran 10 trades, its highly likely that less than 7 would be winners: but if you ran 1000 trades, its very probable that 700 or approximately would be winners. The more trades you run, the more true the probability figure.
The same principle applies to the risk of ruin. If its only 1%, so that 1 trade in 100 would be expected to cause you ruin, then if you only ran 10 trades, it would be very very unlikely that 1 of them would cause you ruin. But if you ran 1000 trades, the probability of a ruinous trade being one of them becomes much greater.
You can only hope that by the time you reach the ruinous trade,m you have already made so much profit that you have actually withdrawn enough money from the account to live on if you should be ruined, i.e. your account is wiped out.