Here is a link to images of elephants standing on their hind legs.
Many are captive Asian elephants (Elephas maximus) trained to do so.
One large male Asian elephant in a circus was trained to walk for hundreds of feet on his hind legs, with a woman standing on his tusks.
Some of the images are of wild African Bush elephants (Loxodonta africana) standing on their hind legs to reach high branches with their trunks. This is done by big old males, the largest elephants. I expect that sometimes the tops of their heads reach about 20 feet (6.096 meters) above the ground.
Of course elephants don't and can't spend all of their waking hours standing on their hind legs.
The anatomy of elephannt like creatures would have to be rearranged for constantly standing and walking to be practical for them.
Giraffes sometimes get up to about 18 feet (5.486) tall, and their heads would be a little higher if they stood on their hind legs only.
So I can imagine a mammalian being that looks vaguely like a cross between a giraffe and an elephant standing on its hind legs reaching and exceeding 6 meters in height.
This site claims that some quadruped dinosaurs could raise their heads to great heights. In the extreme cases 70 feet (21.33 meters) and 84 feet (25.63 meters)
In some cases they might have confused the horizontal length and the maximum height.
But certainly some sauropod dinosaurs were orver 20 feet or 6 meters tall when on all fours. If their bodies were redesigned so they walked on their hind legs, and their front limbs became arms, and their overall weight greatly reduced, such creatures could reach very tall heights.
And on a low gravity planet which has a high enough escape velocity to retain a dense atmosphere, the lower gravity would help with some of hte problems with height, and the tallest bipeds might be considerably taller than the tallest possible on Earth.
I have suggested ways to possible create very large, tall, and heavy bipedal beings. So very old orks could have those body designs and survive to be very large if young orks also have those body designs. If only a few orks live long enough to need those body designs to survive, it would be improbable for the ork species to evolve such body shapes.
If only very old and very large orks get to reproduce, then the body plan of those very big orks would be the body plan of their species, even if it was not so helpful for young and small orks.
Unless, or course, the shape of an ork body changes over time. Most mammals change theri proportions a lot between birth and cessation of growth. Which is why humans can recognize baby mammals a lot of the time. They look infantile.
And even though most mammals have rather short gestation periods and many newborn mammals can already walk, the larger and more intelligent mammals tend to have much longer gestation periods, and even then their babies are less developed compared to adults.
Since your orks or Diremen are intelligent beings, there may be as much difference between their newborn babies and their final life stages as there is in humans, or even more so. Thus the proportions of Diremen will not only change as they grow from babies to adults, but also change after adulthood as they continue to grow.
This changing of body proportions might enable theoldsters to grow much more than if they kept the adult body plan and proportions unchanged as they grew.