It does this because for almost every case it's the right thing to do. People want to *use* the drives they install, and so for the overwhelming majority of purposes Ghost is used for - in-system drive upgrades, deployment, etc - making a disk image use the full capacity of the disk is the desired outcome and thus it's the default both in the manual UI and in automated use.
By default the resize mode is to try and pro-rata the distribution of any change in disk size to all partitions, but this can be changed to prefer particular partitions or to disable resizing.
The command-line references (in the manual or online) describes the switches to control the final disk layout. The simplest is -szee which just prevents resizing, but you can also include the SZE options in the clone switch syntax to control the layout at the level of individual partitions just as in the GUI.