I don't understand the mechanic of making it work on a fixed LY distance increase when everything else in the game about FSDs is about mass management.
I mean, it adds the same jump distance to a 7A FSD as it does to a 2E, and it'll do that whether you ship weighs 50T or 1500T.
I think Frontier wants these to benefit the heavier ships more than the small and medium ships. In other words, it’s designed primarily for combat players flying heavily armored and weaponized Cutters, T10’s, and Corvettes, ships packed with heavy A rated modules and high mass armor, ships that can easily spare a size 5 slot. These ships wouldn’t get as much benefit from the normal FSD boosting by increasing optimal mass (due to how high mass they are), thus Frontier is breaking their own FSD game rules and circumventing the math the game uses for jump range.
Don’t try to make sense of it or reason it out, you won’t be able to rationally. Kind of like how we sit in our SRV’s but not in our SLF’s, or how we need to physically pilot our ships but others can telepresence across the galaxy to join us. Or how wing mission givers will only pay a fixed sum to one player but readily quadruple that sum for a wing of four, for the same amount of work. Frontier isn’t very concerned about keeping consistency in Elite’s world, and they will break their own game rules happily in order to implement something they want to. It might be bad for some people’s immersion, but I really don’t think Frontier is too concerned about that.