The GU-11 fires a 55mm round. Even with advances in technology, there is a limit to how small you can make the barrels, feed mechanism, drive motor and ammuntion storage, i.e. the size of the gun mechanism and working parts is dictated by the size of the round it fires. And don't forget that the GU-11 also holds 200rds of ammuntion.
As for the extendible stock, it's probably supposed to be just like the stock on real rifles, to allow aiming from the shoulder.
The size of the GU-11 is actually quite realistic for a 3 barreled 55mm weapon with internal ammo storage.
One of the many reasons I prefer Macross to Gundam and one of the things I find very unrealistic about Gundam is that there is absolutely no way, you could fit 2 x 3 barreled 60mm guns (or 75mm in SEED!) in the head of a normal size Mobile Suit, the head would just not be big enough, as a 60mm or 75mm multi-barreled Gattling type cannon would be huge, far bigger than the head of a typical MS. 12mm or perhaps 20mm rotary guns yes you could fit, but nothing larger.
As I said, Kawamori knows his stuff and gets it as realistic as possible within the constraints of a giant robot show. Gundam designs like Okawara just wing it and make up stuff that doesn't make sense, but just sound cool.
Graham