iTunes preferencess General > Import settings.
Set it to 32 kbps AAC, then OK. (I'd also set it to Mono if you do not need/want stereo as the file will be 1/2 the size of stereo).
Select the files in iTunes, right click - Convert to AAC.
This will create a new copy of the file in a "compressed" format.
You can then delete the original, uncompressed file in iTunes.