External Monitor & Resolution Problems

When in the office, my laptop is connected to an external display. My two screens are configured as follows:

Laptop (Primary): 1920 x 1200
External: 1680 x 1050

When the external display is disconnected, the primary display resolution stays the same (I’ve just checked using nvidia!) but the desktop icons and text are larger. This means that icons can move about (despite being locked) and I need to re-order them the way I like.

Can anyone suggest a solution, or steps I could take to identify the problem? It’s not a major problem, but it is annoying.