If you open your Python scripts window and go to Render/IO you'll see at the top a script called Mini-Me.  What this does is export everything in the scene and reimport it at a reduced scale as one .obj.  As a result size is reduced since there are no morphs included for the figure, its clothes, etc.   This is always called yowza.obj by default and is in the main Runtime of Poser.   Everytime it's done it automatically overwrites I believe so it's best to save the prop that is imported in your scene in your library.

I use this for larger scenes usually saving out parts or heavily morphed/clothed figures when I'm fully satisfied with the posing since you won't be able to change that.   Bear in mind that everything in your scene is exported and reimported so be sure to do it gradually as you build up a scene.   Going for one huge .obj would not be advisable.  Most times just doing it for the figures in your scene and turning each one into a complete prop with its clothing, hair and accessories and bringing them back into your scene as simple props can be enough to save you on resources.

As for reducing texture maps I can usually go as low as 50% on most maps, add a bit of sharpening and resave and it makes no difference in the render.   Most maps are overkill anyway for render sizes found in the galleries.   It's only for images designated for print that I would not reduce the map size (especially the skin textures).