In a previous game, after getting a few miniturization (+capacity) technologies, I created a double-constructor ship that contains 2 constructor modules, hyperdrive plus engine, a sensor, and a life support for a total mass of 117 on a cargo hull with a capacity of 120.

In my current game, as soon as I researched hyperdrive plus, I am now able to build this double-constructor immediately. The ship displays in the ship designer as 117/90 mass. But there it is, ready for me to build in my shipyards, and I am able to produce these ships.

I am playing with the opt-in 1.01 patch, on a custom race, single player. Will also submit this as bug report as well.