Hello
First a little correction - df displays the volume size, not the partition size.
There is no straightforward way to specify an exact preservation of the volume size.
Linux P2V conversion is file level and is not meant to preserve the binary layout of the disks (unlike disk based V2V conversions for example). There are many things taken into account that affect the volume size like partition alignment, creating logical volumes (IIRC LVM extent size is not preserved), volume block size is also not preserved. All these little differences and adjustments may pile up and result in slightly different volume size.
I am curious - why do you want exact volume size preservation?
Regards,
Plamen