What do apps designed for small screens look like on tablets?
It depends on how the app was designed.  Android provides means to allow an app to choose from multiple layouts automatically depending on screen size and density, so if the designer cared to, they could provide completely different app appearances for phones and tablets to suit their app to the screen size.

Further information on the tools the developers may have used can be found here.

If they didn't do any of that, then the Screen Compatibility Mode kicks in and its behaviour depends on the version.  In version 3.1 and earlier, if the screen is larger than the app is intended for, it will occupy only part of the screen.  In 3.2 and later, it will expand to fill the screen.