If I vary the -quality parameter, the image size doesn't change much, but if I remove the colour profile information my 28KB thumbnail drops to 2.5KB as expected.
An embedded profile in a thumbnail is really not necessary, however the CR and xowiki code is designed to work at any image size. I wonder therefore if it be handy to be able to pass through to the image resize proc whether or not to remove the color profile. Certainly, this could then be supported as a parameter in the yui-carousel includelet.
My version of ImageMagick is old and so I have to do:
convert +profile '' -quality 45% in.jpg out.jpg
For newer versions you can use the -strip parameter.
I will try compiling GraphicsMagick as Torben suggested because ImageMagick won't compile on my (rather old!) linux distribution.