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.
Only against scattered damage ( and due to size not due to thickness ), against a focused laser beam that can cut the armor in a single shot ( for example the destroyers lasers
) it will only provide 25% increased protection.
Real battleship namesakes had vast amounts of armor.
For example the Yamashiro: Total Armor Weight- 12,199 tons (42% of total). So the armor on this Battleship weighted about
4 times as much as the entire weight of a typical destroyer. The Yamato was about twice the size of the Yamashiro and I doubt it had much less weight percentage armor meaning armor alone on the historical battleship should be about
8 times the full weight of a destroyer.
And when it comes to big guns, it's a bit of shame none of the Guns have calibers independent of research like missiles have...
I would love to be able to put huge but crude and slow reload guns for close combat jump-point defense/assault devastating initial barrages.