I frequently reduce texture sizes when applying them to imported OBJs in Bryce.  I too can reduce a lot of textures by 50%, especially for small or simple objects that don't really need a 4000 X 4000 texture map.  As Kalypso said, skin textures, as well as other intricate texture maps, or maps that are going to be seen close up, probably shouldn't be changed.  Other things you can do to reduce texture overhead in Poser or Bryce are:

1. Substitue materials for texture maps.  Glass, metals, stone, etc. look far better and consume less memory overhead than a picture of glass or a picture of shiny metal.

2.  Bump maps, transparency maps, specularity maps, etc. are almost always color pictures pretending to be black and white (greyscale) images.  Change the mode from RGB to greyscale so that they are truly black and white images and you'll reduce the file size considerably.

3. Photoshop, and I imagine other photo editing programs, also has a "save for web" feature that will reduce a greyscale or RGB image even more without any discernable loss of detail.

 

I will often make a new "LowRes" folder in a texture's folder and run every texture through the "save for web" function in Photoshop, reduce the resolution of (sometimes every) texture, and convert bump and transparency maps to greyscale.  It's astounding how small the files become.