Sean Sedwards GIF Slimming Regimen TIP: Many of us may use the Max colours value on the GIF export dialog to reduce the file size, especially on simple designs. It's also possible to make significant savings on complicated images with little loss of quality by combining a reduced palette with the Error diffusion dithering option. On closer inspection however, it appears that Xara X isn't doing all it can to reduce the file size, specifically when the number of colours chosen is less than or equal to half the maximum available (i.e. <=128 for 256 colours or <=8 for 16 colours). Here's the workaround:
Simply copy and paste your artwork to a single frame in a new animation document and export from there. The animation export options don't allow for sophisticated palette manipulation, but you can choose the number of colours and select transparency. If you need to manipulate the palette, make a bitmap copy of your drawing and export that from an animation document.
Explanation: Although the normal GIF export uses a reduced palette, the file still contains the maximum number of colours, each taking 3 bytes (e.g. a 64 colour GIF will contain 192 x 3 = 576 bytes of unused palette). What's worse is that the palette is not arranged for maximum efficiency, so the compression is less good too. You can use any values but the GIF file format supports palette sizes in powers of 2 (2, 4, 8, 16, 32, 64, 128 & 256) and the animation export takes full advantage.
Addendum to the above TIP: In theory, the optimum quality / file size ratio is achieved when the palette size is a power of 2, however I've discovered the animation export has its own quirk (a bug really). If you choose 64 colours, say, and transparency, the GIF has 63 foreground and 1 transparent background colour, as expected, but the file has a 128 colour palette! It's still smaller than going the normal route, but you can improve it further by choosing 63 colours (generally, (power of 2)-1 ) or deselecting transparency.