Sorry for replying late. We investigated the same thing found out the following facts.
In Acrobat 9, display size of every stamp menu item was different which gave a very inconsistent view to the stamp menu hierarchy.
We changed the display size of all the stamps in AX so that menu items in Stamp menu are of same size. We need to bring all the Stamp items to a common size to achieve this with maximum and minimum width and height scaling fixed for them.
Due to this, large menu items will decrease in size like some menus in "Dynamic Stamp" category and display size of small stamp increased like stamps in "Sign Here" category.
So, this seems to be by design. Please let us know if you are facing any other issue than the above mentioned thing or any other of your workflow which is causing issues for you regarding this stamp sizes.
Thanks for your patient and support.