If you enabled a Battery Saving option, yes, it decreases the resolution of the screen (if enabled), which might also cause some icons to appear larger, but turning that option off should bring it back to normal.
If you don't have Battery Saver enabled, then you can check your resolution and icon size settings in your phone's Settings. Just to to Display, and adjust the SCREEN ZOOM AND FONT options to your liking (most likely the Screen Zoom option is what caused it). You can also adjust the Screen Resolution if the above didn't fix it.