Side Note to this thread:
I used a motorcycle battery in my Spark EV for about a month. (I was doing a proper Cap Check on the stock battery, which tested great. It's a '14.)
I made up some short adapter cables. It was a sealed lead acid, I don't think it was considered an AGM.
The differences are minor between the two 12V battery types.
The car only needs a little current to boot up the car, then the 12V is just along for the ride.
I think it's the size it is because of some fed spec to keep the emergency flashers and/or headlights available for X hours.
Plus it has to power the two RF receivers for X days if stored a long time.