Hello Amy,
This is because the resolution on the remote computer screen is lower than the resolution on the Viewer screen, and you are viewing the remote desktop "pixel-for-pixel". You should enable the stretch mode to fill the entire desktop on the Viewer side.
Right click on the connection item in the address book, select Properties -> Modes -> Stretch.
Note, however, that this way you are simply stretching the image, not adding more pixels (graphical information) to it. So the image may not look as crisp as you would expect if the remote resolution were the same as the local one.