video aspect for ipad vieo

Hi

I want to record video on my ipad to use in green screen software.

I recorded the video ensuring all the background was green on the ipad screen, however when i took the video off the ipad the video aspect was wider reveleaing non-green background.

How can i avoid this when filming video with the ipad?

Why is there a change in the video aspect from the ipad screen to the video being played in another media player (I.e VLC).

Many thanks.