Are "use small icons" resized 32x32's or are they separate icons altogether

Hi,

I'm creating icons for a Mac application for the first time and I have the following question: When "use small icons" is OFF, a 32x32 icon is displayed. When "use small icons" is ON, are the 32x32's dynamically resized by the OS, or are they separate icons altother? If so, what size icons are they?

I need to know whether or not I need to develop two icons for each toolbar button.

Thanks,

Don