Smaller...NO...clearly stated same SIZE.
Lighter....why not?
My ASSumptions:
TYPE = lead/acid
SIZE = series #
VOLTAGE = 12v vs. 6v vs. 18v vs 24v, etc.
Some may argue that size implies something other than physical dimensions. SIZE is not defined in the GCR. So who knows who's right unless you are protested and it goes through the process.
If you find a battery that is the same series (size) that has a lower CCA rating, it will generally be lighter. The size and number of the plates inside are smaller and/or fewer.
The intent was probably to prevent people from substituting smaller and therefore lighter batteries. Did they even have other types of automotive batteries? Was it assumed that in order to have a lighter battery back then that it would have to be smaller, therefore weight was not addressed?
I seriously doubt that batteries from various sources for the same application all weigh the same. For that matter all series 42 batteries can't be the same EXACT size.
Put in a cheap 24 mo. Low CCA battery for your application and you'll probably never have to worry about it. Hollow out your battery, insert a 12V lawn tractor battery in it's place, connect the terminals inside the display battery and you could be asking for trouble. I didn't see anything in the GCR/ITCS that defines the required distance between the plates and the outer case.