After a couple of tries and reading your notes, I finally managed to reproduce this.
The problem here is the poser primitive, or rather the scaling that is being done to make it fit as a floor for the walls.
As you’ve found out, making a floor in carrara, or any other modeller, will work much better,.
I’m not aware of any products which make use of poser primitives,.. apart from example scenes supplied with poser.
and this is probably one of the main reasons for that.
Poser isn’t a modeller, and it’s probably not a great idea to Mix modelled objects with scaled primitives.
In Carrara,, (motion tab) the scale of the loaded prop from poser, shows that the whole object is being scaled wrongly, presumably because the “Parent” object, is the “Scaled” Box primitive.
In Carrara, you can build a simple floor or ground, or even a small terrain, and export that as an obj,
then you could export the buildings, as individual objects,. then assemble those in poser as needed.
Primitives are primitives, and any “virtual” scaling that is done on a primitive, is only virtual,.
to edit the shape of a primitive it either needs to be edited in a modeller, or exported (from poser) as an obj and imported again as a model. not a primitive.
The same issue can be seen in Carrara,.. if you use a Primitive,.
for example,. if you create a Primitive cube in carrara, then scale it,.. then Convert that to a vertex object (edit / Convert to other modeller / vertex modeller) then in the vertex modeller, what you’ll see is the “original” cube, without any scaling, since the assembly room scale of objects doesn’t effect the “real” scale of the model.
to change the scale / shape of a model, it needs to be edited in a modeller.
Hope that makes sense