Keep a frittata puffy 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?
You may want to see Serious Eats' coverage of scrambled eggs.

Getting them to puff initially requires a little higher heat.  Adding extra liquid (water, milk, cream) will result in softer eggs, so you may want to reduce or eliminate it.

As for the cheese -- it really depends on how much you're adding.  If it's so thick you can't see the eggs under them, I'd recommend either reducing it so it's a bare sprinkling, or mixing some in with the eggs and then only a small bit on top.

For a fritata specifically, you should be careful about not overcooking it -- it can cause it to deflate if the air bubbles over-expand and burst.

When dealing with serving frittatas at room temperature or chilled, make sure that you let it cool slowly -- you need to make sure that the proteins get a chance to set up before the air bubbles shrink significantly.