If the Get is creating the toFile it may not be creating it with the same numeric types/sizes. Check the DSPFFD.
The Put file might have deleted records which will not show up on record counts but will take space
If the Get is creating the toFile it wouldn’t have the keys of the FromFile – that takes space.
I’m sure smarter minds will add to this list.
When I did it, I only compared record count.