It was probably the image file to begin with. This past spring, one of the woodworking magazines did an article on SketchUp and offered as an "online extra" some wood grain images. They were nice images but way too large to use as is. Use one or two of them in your model and the file size would jump considerably.
Thanks again to all who responded. To anyone else who wants to learn about layers and scenes in woodworking design, I recommend Joe Zeh's Beginner's SketchUp Tutorial Series http://www.srww.com/google-sketchup.htm and especially his "Rules for 3D Drawing Using SketchUp," found in Part 1. There's also a good description of converting JPG files to textures, and orienting and aligning them to components properly, in Part 8.
