I think you may be wrong there - I thought all bridges care about the mass of the ship being bridged?
Anyway, Stront is a flat cost for opening a bridge. This means Stront as a baseline cost is mainly relevant for opening a bridge for a small amount of low-mass ships, not for the high-mass ships, as the stront cost for bridging high mass ships will be relatively less compared to the fuel it takes to bridge heavier ships like freighters.
Cost to bridge = Stront cost + (Fuel cost * mass of all ships combined * distance * some constant)
Without Stront it would be essentially free to bridge a single frigate as Titan, or bridging a single Venture as Rorqual.
I would think this is why Stront as a requirement to open Titan bridges exist: to give briding a baseline cost even if the mass of the ships bridged is negligible.
The same would be true for Rorqual bridges (when not conduit jumping), so I don’t see why these should not have a similar Stront requirement to open a bridge.