Theres something really weird going on with class design, Tonnage randomly changing when altering class design, adding armour, or even just checking the "show tonnage button"
Just clicking on the class design alters its tonnage.
I click "new ship class" get a 364 ton design.
Hit "copy design" This design is 404 tons, click on the old design and now its 392 tons, if i disable "size in tons" now its 404 tons, reenable it now its 408, if i turn on no armour it drops to 172, disabling no armour now its 299 tons.
increase armour to 2 and now its 565. Change the armour to 2 again, now its 775 tons.....
Edit: Clicking no armour at least gives a consistent size, and also when you turn the armour back on again.
Edit: The You also get a consistant size if you change the armour only once after selecting Zero armour then disabling it again.