It's my guess, that your scanning one of the pictures in at one resolution and the other picture at a different resolution. If you do this, then you will see 25% or 50% because there is less or more pixel information then the other.

My sugestion would be, scan them both at the same resolution. If there different size prints to start with then the 25% or 50% will be different no matter what. Refer to the manaul that comes with photoshop for info on how resolution affects size or dimensions of a photograph. Good luck

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Wait I'm not done yet! Ok, now I'm done.