Not unusual in ships. Big ships can be sunk by one or two well placed torpedo's.
The biggest problem with the Graf Spee was finding it. After it was cornered the Germans sank her themselves. It wasn't even combat that sunk her, but false information.
During WWI many British ships were destroyed because German shells ignited gunpowder. One (un)lucky shell and down went the ship. Turns out the powder was being stored in the wrong place.
All it took to sink the Titanic was one iceberg.
Not to mention, apparently "absolutely unprotected" doesn't take into consideration the hundreds of Turbo Laser turrets and thousands of TIE fighters that the Death Star had at its disposal.
Oh, and the whole planet destroying super laser. That was kind of a big deal.