Its is perfectly plausible that a creature can be 9ft tall.
Larger circulatory system, adapted organs, perhaps more than one heart, or one single very powerful heart with multiple chambers. Orcs could also be like the avatar people in, well, Avatar. The Pandorians had carbon fiber naturally embedded into their bones. Perhaps Orcs have something on that line, like an especially large amount of calcium that they obtain from the prey in the desert. In response, orcish muscles would also be able to support these bones.
Thinking back to my comment on the 'prey of the dessert', there is some pretty big prey ot there that could supply calcium and protein for the orcs so they could grow? Scaddernaks, for example, an animal that lives in the desert, is gigantic. They are much bigger than orcs, and could easily supply the necessary amount of food. Orcs could have easily evolved to be able to match these creatures in size and strength. Lur Bulls. . .I played one of those when i was in the ET. They could easily provide protein for the orcs, and the female counterparts could provide milk. I am sure orcs would learn to farm these animals.
Who said that orcs have to live in the desert? Now they live in a savannah. They've lived in a desert historically, but could have easily moved around and obtained food like nomads, which I would expect would be how they lived in the beginning of their days. The only orcs that probably lived in the desert were the shamans who believed the desert was connected to the spirits. The Shamans, being very important, were probably fed by other orcs, who brought food from abroad. This great movement from biome to biome hauling food and materials from their nomadic villages into their main cities could have built musculature, and could continue to do so today. After all, raiding is nothing new to the orcs, and it takes someone plenty strong to haul loads of loot back to their base.
Perhaps evolution decreased their brain capacity in order to dedicate more energy into body growth, as advanced thinking was not too important in a high survival situation like the arid climate the orcs evolved in, explaining the orcs lower or varying intellect compared to the other races.
Another extremely plausible theory is that orcs discovered agriculture, and they started manipulating oasis into farming sites, or even perhaps created their own artificial oasis by channeling nearby river water into places. Most probably their settlements were near freshwater or in oasis anyways, and they used the land around the rivers to farm, like the egyptians.
There is a million possibilities to explain the height of a beast like the orcs. Now, orcs walking into buildings with 6 foot tall doors? That's the player's fault, not the races. Orcs are fine as they are. We've had populations like them IRL as well, for example, as i mentioned before, the Egyptians and Persians. Both of these were great civilizations which sprung up in the most remote of places, but through agriculture and terraforming managed to develop into the world's most prominent civilizations.
This is what i consider an accurate depiction of an orc:
And, one of an olog: