There is a tradition to make very big Ger in Mongolia, so big that one-piece Un become impractical. The solution is to insert and extra Tonoo in between. This Tonoo rests on its own Bagan, more or less halfway between the central (sometimes VERY BIG) Tonoo, and the Han. With the growing of the Ger, the Han also grow, to as much as 3,50 mtr high. Dimension of lumber grows, and the Han are not extended to 90 degrees, but much less, something like 60 deg.
These big ger are not moved much, all of them that we visited have poured-into-concrete stiffening upright poles, vertical, inside and as high as the walls. Bagan are sometimes fixed to the concrete, sometimes not.
Avtai-Khan-Ger is the most known in UB, it has been up for at least 15 years. It measures about 25 mtrs across, and is used as a restaurant. Wall-height about 3.50, diameter central Tonoo 5 or 6 mtr, about 462 un, but I was not able to count precisely.
Centre-height maybe 8 or 9 mtrs?
BIG stove in the centre, melted a litlle... It seems it carries multiple layers of felt, as if added over the years? Avtai-Khan-Ger is in good shape, after all these years.
An entrerprising gentleman put up this Big Ger as a central restaurant in his Tourist-camp in front of Erdene-Zuu monastery in Harhorin, in 2001. The camp had to move to make place for a commemorative monument of Unesco, so this big ger also moved. We visited again in 2006, it has lost a lot of the splendidness it had in 2002.
In this Ger the inner and outer Un are different length, but the decorations are the same. The central tonoo rests on 4 bagan, the outer tonoo on 12. Centre tonoo is maybe 5 mtr, outer tonoo 12 mtr diameter. Total diameter we guess at 20/22 mtr.

There are twice as many outer-Un, as inner-Un.

Outer Tonoo is built up of three layers of massive wood, with conventional holes drilled/chiseled for the outer Un. Un sizing at top is about 5 cm square. Big Tonoo seems not to have any clever system of taking apart, it is full of bolts and wooden pins.
Inner Un seem to be resting in/in the big Tonoo just like the would in a conventional doorframe, half-a-notch. We could not find a ladder, or permission, to go up and check....
In Gandan Monastery in UB, recently a new BIG GER was put up as a temple. It is obviously made by Ger-Tereg, as seen by the routed/chiseled decorations.
It is 18 mtrs across, 2.50 high on the walls, has four/12 bagan, inner tonoo is 5 mtr, inner and outer un are the same, and there are as many inside-un as outside-un.
It is a bit of a budget-ger, the Han are flimsy, and somewhere in the process of making the wall-spacing was raised from 30 to 50cms, making an interesting emergency-solution neccesary to rest too many Un on too Little Tologh.
It sports three doors, to reinforce the too weak han, says the man there. 16 Tologh on each of six walls support 24 Un. Total of Un is then174+174.
The vertical purple-cloth-covered poles are scaffolding-pipe sunk into the concrete ring that surrounds the Ger om the inside.
Nice detail: Non-rectangular carpets, specially for in Ger!