What are the steps for pouring more cement around metal fence posts that are leaning?
I live in a "high wind area" and you don't need to go with deeper holes. 

I think your 30" depth is adequate and the size of the hole is more than adequate for a 6' fence. The problem is that you need to pour a concrete "collar" at the top of the hole. 

I guess you could fill the post hole up with concrete, but you have rather large holes and that could be expensive. I'd dig out the dirt backfill down to the concrete at the bottom of the hole. (I'm assuming the metal fence post is partially encased in the concrete at the bottom of the post hole.) Then, I'd fill the hole up with crushed rock to the point that about 1 bag of concrete would fill the balance of the hole...creating a collar at the top of the post hole.