What is your db option for forms - tabbed or overlapping (go to file options, I think current database, to check). If set to tabs, then any new form will fill the screen but your old forms might be set to popup, which will override tabs.
Perhaps more likely there is a macro or code which invokes Maximize when the form is opened? I don't see how adding data would size a form to full screen when it doesn't do that when there is no data - as long as you're not going in to layout view and not form view.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.