October 2017
Hard to say what might cause the issue with so little information.
What device are you using?
Is it a phone or a tab?
Android or iOS?
What OS do you have? Marshmallow/iOS11 etc
One thing that will work is if you get a bigger device, I mean a +10 inch screen is easier to view than a screen that is smaller than 4 inches.
October 2017
I'm guessing you're using a Samsung device. A lot of Samsung users are reporting this problem. You may be able to adjust it with an app called Game Tuner For Game Launcher made by Samsung Electronics. It doesn't work on every Samsung device though.