Okay, I don't know about bulk, and I don't know about the quality of the output, but you can tell automator to amalgamate all of your A2 pdf's, open it in Preview, then in there choose your paper size, then at the bottom section, select 'Scale to fit each paper'. If you have problems with the margins scaling properly, you might have to make a custom paper size. Then save it as PDF at the bottom left menu.
Fire up that new scaled PDF in Adobe Acrobat, then separate them all out again.