Yeah, I should go into more detail here.
Ship mass is pretty much as it says, the
total maximum mass of a spaceship. Every module, every reactor, every fuel tank, everything requires mass. This has side effects on things like maneuverability, the ship's structure (pushing a vessel to a combat speed of 10 Gs puts a
lot of stress on a 950,000 kton starship), and the ship's ability to take recoil to name a few.
Ship Volume and Surface Area utilize the same data (specifically length, width, and height for the most part) but in different ways. Now Volume is how much giblets you can put into a vessel, which includes storage spaces, crew facilities, ammo bunkers, among other things. Some weapons are
very volume intensive (VLS and torpedo cells anyone?) while others aren't.
Surface Area is for all the external bits and heavily determines how much your armor weighs. Having a starship with a surface area 87,500 square meters is going to have less mass to it's 5 meter thick armor plate than a vessel with 175,000 square meters in surface area. Now going larger is a necessary evil due to what I'm thinking of when putting external giblits onto a design. Now more surface area is a necessary evil for putting larger turrets or turrets with multiple barrels onto a ship design... and this is partially due to how armor works, which is inspired by Sword of the Star 2's armor model.
(the inspiration: Sword the Stars 2 armor 'matrix', the green bars represents armor you can ablate away while the blue bar represents 'Armor Resistance', which can no-sell various weapons outright if you play it right, and further enhanced with certain technologies or upping in ship size)
Basically armor has this attribute that no-sells a certain grade/tier (based on being on a certain class, although I haven't completely decided on the specifics) and below. Basically a weapon of -for the sake of argument- the 'small' tier can damage/penetrate 'small' tier armor but is no-sell-ed by 'large' tier and up armor and the weapon's capabilities are halved when facing 'medium' tier armor. This means in order to slap ships with weapons that can harm vessels within a certain grade/tier of armor, you'll have to sacrifice a
lot to get that weapon fitted...