Video playback always zooms the first time

[AC Question]

A few days ago suddenly whenever I play a video for example from whatsapp, the video turned on in zoomed condition (blue box of zoomed video in front of the full size original video). I had to manually expand the video to bring it to full original size. My phone is Samsung galaxy s5. How do I turn this default zooming off? Any help is appreciated. I have read about magnification gestures, easy mode, none of those worked. Thank you in advance.