My frittatas always deflate. I've heard all kinds of differing advice: add flour, add tapioca flour/starch, add corn flour, add baking powder... I was once even told to add a little yoghurt! The base recipe I use calls for 6 eggs and 1/4 cup milk, and then some onions, garlic, and then a little of whatever vegetables are in the fridge chopped up fine... and then shredded cheese on top once it's already half done (but I don't think the cheese is so heavy that it's causing the whole thing to collapse and deflate - I really only use a sprinkling). What's the best way to keep a frittata from deflating? 
If too much water is the culprit, then its a simple matter of reducing the soup on the stove top. Just put it in a pot and boil it with the lid off(so the water escapes) until you reached a desired thickness and flavor. 