The whole "fighters to be launched" stuff is well and truly in the dark atm, and if introduced would assumedly run off fighter bays with classes if they need different size ones.
Hull mass contributes to your overall mass, which affects things like handling, but the main one is it affects your shield strength. The higher your hull mass is, the weaker the shield you get. Heavier ships will generally have the bigger internal spaces for bigger shield generators to compensate, but still explains how the shield strength is reached and in some cases gives unusual results. For instance the iCourier has a class 3 internal slot max, but a class 3 shield generator gives it a well known beefy shield because it's got a tiny hull mass.
Notably only your base hull mass makes a difference. Any weight added in the form of weapons, fuel, internals etc. don't touch the shield strength. The idea is that you're stretching the shield over a bigger ship, but they haven't implemented a "surface area" stat and use mass to compensate. Hopefully this isn't something they try to add later