As an engineer, I can tell you, mistakes of that magnitude don't need to be explained. Sometimes, they just exist because a moron fucked up.
Pre-Rogue One, I bet some Imperial HVAC designer say "The exhaust port HAS to be 2 meters wide! At the airflows we're dealing with and the length the duct has to be, with all the blowers it'll take to vent the damn core, if we make it smaller the whole thing will overheat and explode!"
And the Project Manager was sick of dealing with HVAC's shit, so they never got together with the Defensive Design Consultant Engineer to make sure it wasn't a problem.
THAT'S why the port failed the Proton Torpedo crosscheck.
I love Rogue One because that explanation is better and more fun than what I just wrote--even though what I wrote is more realistic.