I've created images for taskotron using both taskotron-vmbuilder and imagefactory. They're similar but not identical - I based the imagefactory off the F22 cloud images instead of specifying the server group install and virt-builder has some restrictions on what you can do with disk space which imagefactory does not have.
I've put all the files up for review: the kickstart for imagefactory, the yaml file for vmbuilder and both created images, gzipped.
https://tflink.fedorapeople.org/taskotron/testimages/
Time of creation operation
imagefactory: 16m6.596s vmbuilder: 7m19.273s
Image sizes
20151113-taskotron_server-22.qcow2.gz 818M 20151113-taskotron_server-22.qcow2 11G 20151113-imagebuilder-taskotron.qcow 3.0G 20151113-imagebuilder-taskotron.qcow.gz 358M
I hoped you would compare these two approaches with the same "disk recipe". If you create a 3GB disk and install heavily stripped cloud package set using imagefactory, and then create 10GB disk and install full server package set using virt-builder, there's no point in comparing time or sizes. Could you please try both when e.g. having a 10GB disk and installing the server package set?
Also, were you using "ls" when printing file sizes? That doesn't work for qcow2 images, they are sparse. You need to use "du" for that.