its not even the full plans leaked to the rebels either. Daedra puts together that Krennic is building the death star, but Jung mentions that he didn't see everything, there was too much. He knew it was a weapon, uses Khyber crystals from Jedda, kalkite from Ghorman for panel coating, the design was refined on Eadu, and Galen Erso was the lead designer.
its not really until Rogue One until they finally realize its a moon sized planet killer.
It is pretty funny to think about it in universe though, the Emperor and a tiny inner circle keep it a pretty good secret for over a decade. But its destroyed within days of the Rebels figuring it out.
Then for an insanely bizarre reason the Emperor decides to build a second one, that is blown up before its even fully built out. It still can be blown up by a few good shots in the core which now apparently has a million giant tunnels leading to it. Lando fires a few shots at a core reactor and the entire thing blows up. That seems an even worse design than the first one! The only big design change is that they put a force field generated from the nearby planet to protect it from starship assaults, but then I wonder how the fuck would the Death Star move around the galaxy? It can't tow Endor around. Without Endor's force field anyone could fly in and blow it up.
Then the worst of all, in RoS we learn the Emperor afterwards has been building a fleet of 50,000 starships each with a mini death star cannon on it on Exogul, but those are all destroyed within 2 hours by a rag tag group of randos because all the ships share autopilot or something?
Seriously the Emperor hires the absolute worst engineers in the galaxy. If the rebels did absolutely nothing I'm pretty sure every imperial superweapon would basically start accidently fragging friendly ships and self destruct from power overload.