After this market closes, I will add up the numbers of log(shares) held by each YES trader and each NO trader. If the sum is larger for the YES traders, this resolves YES. Otherwise (including a tie), it resolves NO.

Now the the benefit to having more traders on your side, as opposed to having larger bets, will be even more pronounced, to the point where it is basically impossible to manipulate the market by making large bets (though other forms of manipulation would still work).