The size of the new ISO depends on the parameters you use within genisoimage and for create_compressed_fs. Especially for create_compressed_fs I use the standard compression level; if you select "-b" (try all an choose the best compression method) the used time to build a new ISO becomes tremendous.

For the old chroot-method for remaster you dont't need VirtualBox; I only suggest it for the kn-remaster script.

Greetings Werner