Any imaging tool overwrites the partition table and erases any partition or structure which may exist - although it will leave unallocated space.
The partition is expanded on first boot by the Pi itself not by the imager.
The exact mechanism used by Raspberry Pi Ltd. has changed in the last couple of years and I don't remember the details BUT if you examine the cmdline.txt file before booting the Pi you will see at the end a script that is run once and later deleted.
You could delete this BUT the current first boot also creates the user (it probably modifies pi) so the image may be unusable (until the very small root partition is expanded there is insufficient space to run effectively).
You could examine
/usr/lib/raspberrypi-sys-mods/firstboot and modify it but it is likely to be easier to shrink the newly created root partition, which is simple on any Linux system.