The prototype do not stretch the frame.

Hey ​@Вікторія, thanks for reaching out! 

It’s hard to be sure what’s happening based on your screenshot alone. Did you create a flow using your top level frames? If so, did you also select the frame with your flow starting point before switching over to the Prototype tab? 

If you’ve done both of the following, are you able to share a screenshot of your file including your layers panel and Prototype panel, so we can take a closer look?