Interesting subject.
The best way to reduce the size of the file can be done by choosing File > Save and compress menu item, instead of default save. This can easily lead to a 3 or 4 times smaller file. The compressed mode isn't the default one because it takes also quite longer to save.
And about texture images, it's the resolution (number of pixels) that slows down the performances not the size of the file. So, if you save your textures in JPEG format, prefer a higher quality over a larger resolution.
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Emmanuel Puybaret, Sweet Home 3D creator
