Search results page numbers are usually (not always) meaningless. What's meaningful are the results that the user sees, so that should be the anchor. After recalculation the user should end up on the page that displays the result that she was looking at when she initiated the recalculation.
In both cases they are in the middle of going through the results, where they reached a specific point. After the change they need to be able to easily pick up from where they left, so it's important to give them that anchor.
Since you can't really know for certain which result the user was looking at, you will often have to guess where to take them after the change, especially when the change is to reduce page size. I think that going by the top result on the page is the safe choice.