mobiles different screen sizes

Hi, I'm trying to build a game for Android, but I'm having a problem. different screen sizes (ans different aspects). When I run my game on unity everything is nice and the view is great. However, when I run the game on mobiles, I have a problem that sometimes the scene gets something like 'cut'. What I mean is that sometimes the view is bigger than it was on unity, some other times on a different mobile it's smaller than it was on unity. I know it might be a generic question, but I have really tried to fix this and looked for similar questions, non of it help. Can any one help me here? this image is how my game runs on unity :

this image is how my game runs on a mobile device :