The size of the cannons threw me off at first, until I did some digging. There are 2 designs out there: The Devastator and the Avenger. The devastator was made for A New Hope (and remade in CGI for Rogue One), the Avenger was made for Empire Strikes Back (thought the Devastator was using in some scenes in ESB). Almost all the star destroyer pics and models out there are based on the Avenger, which is a much larger model, more detailed, and more complete model. There are 3 primary differences in the designs: turrets, radar, and the garbage door. From the films it is actually very difficult to see the difference, because most of the shots of the star destroyers are looking up at it, causing the cannons to be out of view (or so small they cannot be seen/discerned).
The Avenger cannons are much smaller and each cannon turret has 4 cannons on each side. The Devastator has 4 large cannons on each side that look like WW2 tank turrets.
The Devastator has an X-shaped radar that sticks upright on the bridge between the shield generators. On the Avenger, the X-shaped radar lays flat on top of the bridge.
The garbage door on the back of the Devastator is at the base of the bridge tower. The garbage door of the Avenger is higher up on the tower.
The cannons and bridge are a little over-sized on Charles Andersons Intimidator (which was modeled after the Devastator in Rogue One primarily), and he explains that is to have extra accuracy in the details.