Reduction are the same size as increases, same as for lower doses. The one thing that is different, however, is the size of the depot. The larger depot can influence quite a few cycles after a reduction. To play it safe, one option is to do a reduced dose (or BCS) after a reduction is earned, to try to reduce the depot a bit too. A BCS could be 1/2 to 2/3 of the normal dose, or might not even be needed. But I'd start conservative and see how he responds.
On a somewhat related topic, this is where R can help. If kitty is getting an L insulin + R regularly, the effective dose is larger but the depot is smaller. After a reduction you can just skip the R dose and don't have as large a depot to deal with.