Are these buttons properly flowed into your UI? Check the anchoring and scaling. Put the resolution of the simulator into your Game window and find out.
Here are the official docs on how to make your UI handle various resolutions and aspect ratios cleanly:
Here are some notes on UI Anchoring, Scaling, CanvasScaler, etc:
Usually you need to choose a suitable ScaleMode and MatchMode in the Canvas Scaler and stick with it 100%. Generally if you change those settings you will often need to redo your UI entirely.