my SKB is 39 megs. how to reduce in size? purge? Remove…purge unused components and materials
The SKB is a backuhp file.
I assume you mean the SKP.
Use Model Info > Statistics > Purge Unused…
This removes ALL unused component-definitions. materials, layers and styles…
If you have used any humongous images as textures consider making these smaller files…
Also if you have lots of stuff assigned to OFF layers [or ‘hidden’], then consider if you really now need them.
If not, then delete them…
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PS:
To make your model more responsive consider these…
Using Layers to switch OFF unnecessary clutter when modeling new stuff.
Using Styles to minimize complexity - of edge-rendering, texture-rendering-mode, shadow-settings, fog etc etc…
Using Scene-tabs to limit what is visible while you model other stuff.
There are may ways to optimize your working time in SketchUp 
that worked, thanks…