How to crop image in Keynote or Numbers?

• double click the image placed on the slide to select the crop tool
• with the Crop button highlighted blue, drag any of the images handles to crop
• re-position the image by dragging and zoom in using the slider

Keynote can not do what you want, using the crop tool in Keynote is a non-destructive process, Keynote can not reduce the image file size by cropping.

The only way to have smaller image file sizes in Keynote is to crop and reduce the files in another application before placing on the slide in Keynote.

Thats a different question requiring a different procedure.

To reduce file size, in proportion to the crop being used:

Duplicate the image file to create a back up

Open the image in Preview, located in Applications Folder

click and drag with the mouse on the area to be cropped, then use Command K (Tools > Crop) to crop the image

save the file.