It's not about number of colours, in jpg it's about how fast they change, a 2 colour image that swapped every pixel to create a dither pattern will still be huge as jpg uses a length - string compression. For example, colour the next 100 pixels in x colour, now some of the pixels may have slightly different shades, but they'll be blocked to make the compression more effective, hence the loss.

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.