Imagine that you are selling an option on a 600 stock. A 10% swing in the stock means + or - $6000 loss if you are wrong. Now if you were selling an option on a $60 stock, the loss would only be $600 on a 10% swing not in your favor.
Its one of those things you just have to face with indicies. If you don't want it to eat your margin, buy an of the money option to make it a vertical. I.e. sell 670 call, but buy a 700 call to hedge. Your margin will only be 3K.