In DAZ studio
When you decimate you can select the individual items within the scene and set their reduction seperately. usinng the LOD button actually creates a reduced polygon version which can be selected under the object/figure perameters tab in the resolution dropdrop.  normally there is only "Base"  but if you create a LOD this wil be added here. The LOD does indeed provide for reduced resolution models as a function of distance from camera. if you toggle the resolution you can see the effect a decimation has on the model.

Will Daz allow you to decimate according to "smoothing groups?"
If so, assign the eyes a different smoothng group and decimate only the part you need to.

Guess I am puzzled that if I have a Scene that is 25 Mb saved, and I export it to FDX it's like 1.4 Gb
When I run Decimator I have like 1503030 polygons,  so I change it to 5000  ( was about 5% left)
and hit Done.   Of Course I know it's looks blocky for now. But I am testing.
But when I save it again as FDX then the filesize becomes 3 Gb and 3DXhange refuses to read it then. same as the 1.5 Gb file.
I would have thought that the filesize would end up like 300-600 Mb  but it's not.
That's why I am so puzzles.
When I save it as a new Scene again.  the filesize stays at 25 Mb  

Other than iClone projects, DAZ projects reference the objects in the scene rather than loading them outright. That makes the scene file smaller than the exported file, which needs to contain everything.

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Want your fbx export to show (Only) the smaller size..... Delete everything else, before exporting.

I don't understand.
Everything is in my scene that I want for exporting to FBX.
What am I suppose to delete?
When you export an FBX , isn't it exporting whatever you have in the scene currently?
I tried the settings 2011 and 2012  (whatever the difference might be for the export)  forgot what it said .

Working with LOD... As to not distroy my original.... I generally have a  higher poly and the reduced poly version.
If you export this entire scene your high poly version is still in there... making the fbx larger in size.
If you don't do this.... and you decimate a mesh... how does your export not show the proper "reduced" vertex count???

Well,  That's why I am asking :)

To narrow it down, I'd try this:
Decimate one object, export it using the fbx format... not the entire scene.
Open the newly decimated object in a new scene and it should show the exact vertex count when decimated.
Repeat.

Then open up a new scene and import all of you newly decimated fbx objects.
Note the exact vertex count.... export it to fbx.
Import it again and ensure that the proper vertex count is displayed.

Double  check your import and export settings. Something is increasing your vertex count.

I want to import scenes from Daz to Iclone.
When I did that in the past, I ended up with 1.5 - 4 Gb files. 

if your Character is to heavy, check in the Parameter Tab the Mesh Resolution . It must be set to base, otherwise the Polycount is 4 x higher .
https://www.versluis.com/wp-content/uploads/2016/04/Screen-Shot-2016-04-27-at-22.51.02.png
Another important thing,  decrease the textures . Remember a DAZ Figure have many material  ID's with high resolution texture images, that eat the peformance.
In DAZ is a Texturatlas tool , that shrink many textures  and bring it in a light weight thogether .(single sheets)
https://helpdaz.zendesk.com/hc/en-us/articles/207530243-How-to-Add-the-Texture-Atlas-Icon-to-Studio



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