I'm not sure that I would allow it to increase the size of a construct since generally a construct's size is based on the original item. I would, however, allow them to add HD to the max allowed for the size.
What I've also done is gone the "Runic" route, which is similar to the spell-stitched template you can add to undead. t doesn't add much to the physical mass of the object but you engrave or add engraved materials to the existing construct and give it additional HD and abilities. Other templates could be added to augment the construct and make it more useful.
You might also have some process that increases the HD cap by using polymorph any object to change the construct's base materials to something tougher.