Zynga has strong lawyers. They would force small devs to take them to court, if they could afford the legal fees, then settle if they had a strong case. By then Zynga's game went viral and the smaller devs lost their first mover advantage. The SF chronicle wrote a great piece about these tactics a few years ago. The article showed how they cloned FarmVille, then settled with the victim for 11 million (FarmVille would make a billion in rev that year). im on mobile and can't search for the article rn
It shouldn't be difficult to find someone willing to help you fend off their lawyers if you have a compelling case. Again, if the violation is really this flagrant, the party alleging infringement would be calling the shots and Zynga would give them any reasonable figure to prevent the case from going to trial. Obviously it's not reasonable to hand over a year's worth of revenue.
