I think you should dock some material from both sides of the Ab plate first then resize before removing any from the back plate.
AP armour used to be quite small around the middle, if you take a look at mine, I didn't need to remove any material and i am thin.
Mark re-worked his moulds a couple of years back to make some parts bigger to suit a wider array of body sizes as well as making a few accuracy corrections.
He certainly extended the sides of the Ab plate so you can remove an inch either side of that.
It is good to have some room to move in your armour from a comfort aspect and the gap can be taken up with the strategic positioning of a few pieces of foam so it doesn't move around too much.