Many monitors are set to display at 640 x 480 pixels. You may have gone to a web page and had to scroll from side to side. This means that the page had more pixels horizontally than your monitor was permitted to display. Since your resolution is going up but screen size isn't changing, everything will appear smaller at a greater screen resolution. If you have difficulty reading the monitor because text is too small, lower the screen resolution to avoid eye strain.
Colour preferences can also be different from monitor to monitor.
Some allow you to see millions of colours, others only 256 colours. If you have your colour preferences set to millions of colours, you will see richer hues and tones than if you have them set at 256 colours. It will also take longer to load a web page. These can usually be adjusted using the buttons on the front of the monitor. Older monitors and video cards are only capable of displaying at 640 x 480, at 256 colours.