My take on Gundabad and the Northern territories is that they feed them a lot better up there, and their genetic lines are far stronger than what they are in Mordor. Even their Wargs are bigger and faster than the other Wargs of Middle Earth. During the Hobbit films, we can see Gundabad Orc front-liners leaping and showing greater athleticism than the average Orc or even Uruk-hai, with the exception perhaps of Uruk-hai Berserkers. Bearing that in mind, we can suggest that the Gundabad front-liners are better fed and train more regularly than any other Orc forces we see in the complete collection of films. Perhaps the Uruk-Hai could have been agile and athletic too, but it's possible they were just bred to siege so bulk would make sense.
In any case, as has been noted, yes they're taller than Mordor orcs, definitely at least 6' tall, if not more so. But just looking at where they stand by comparison, they're a highly versatile breed of Orcs.
Goblins/ Moria Orcs are the most agile by far, sleek and spider-like, but are also the smallest and weakest
Mordor/ "regular" Orcs are bow-legged and somewhat cumbersome, lacking agility and great strength, they're just cannon fodder, and vary from Goblin-sized to short man-sized. They strike me as largely underfed slaves
Gundabad Orcs are as tall as Men, stronger than Orcs and Goblins, and have the agility needed to prowl, stalk and also leap in combat. They could accomplish a variety of different tasks, in and out of combat
Uruk-Hai are the tallest and strongest of the breed, able to wade into combat with a physical advantage, but lacking the agility and stealth of the Gundabad Orcs. Definitely the heavy infantry of Sauron's armies, but not as all-purpose as Gundabad Orcs, though Uruk-Hai may still have greater overall stamina, judging by their marching abilities
I'm really excited to see if the Battle of the Five Armies final conflict shows more of the Gundabad Orcs, they're an impressive force. I'm guessing the efforts of Azog and
his subsequent defeat
must have completely expended the Gundabad forces in Middle Earth?
One last point, I think there's a variant of both Uruk-Hai and Gundabad, the Berserker type, an Orc whose courage and almost suicidal determination makes him nearly impervious to pain. Looking at the brand-carrying Uruk at Helm's Deep, Azog, Bolg, Lurtz, there are examples of Uruks and Gundabad Orcs taking extreme punishment beyond their kin's abilities.