PST File Same Size After Archiving

Try compacting it.

I believe the .pst format is a lot like databases in which removed records are "tombstoned" instead of physically deleted. This is done because it would slow things down a lot to rewrit the entire .pst file every time an email was deleted.

So the spaces where your archived email did exist in the old .pst are marked empty and will be used to hold new data, but those spaces weren't removed. Compacting removes the empty space.