If I told you they won't deflate, I'd be lying! They do come out perfectly puffy and collapse a little as they cool because they don't have anything to support their structure other than egg and milk proteins. There are a few ways to keep them as puffy as possible. Make sure your oven isn't too hot (I use this oven thermometer) and let the muffins cool in their tins for 5 minutes before removing them. Make sure not to open the oven up while cooking as this can affect their structure.