Minimally thicker: yes, but the belt itself is considerably wider overall and unless there are some very lucky shots that hit the same area it will provide a lot more than 25% increase in protection.
I was kind of disappointed in the overall armament. It has a diverse array of weapons. I feel like it is going to be mediocre at everything and that the smaller ships that are specialized will end up being considerably more useful. Kind of a 'Jack of all trades, master of none' kind of problem.
I have hit on one "Battleship" (can't do cruiser work) design template that I think works, and ironically it was the result of trying to intentionally make a "pre-dreadnought" design, the proverbial 'jack of all trades, master of none'. The freakish thing about it was it required an abnormally high base tracking speed, but otherwise it did battleline work like nothing else. 2-4 Large railguns, 10-20 Small railguns, 20-40 large missile tubes (hour reload speed, thermal sensors) , 60-80% cruiser speed.
EDIT: Wow the forum did not like something about this post, 5th attempt to make it and I eventually hit on posting a sentence at a time and modifying them in to see what broke things.