Less contrast = less colours = smaller palette is my guess. There are lots of ways you can slightly influence the file size.
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Unless you can find a program to save a TIFF at a specific size, your could try making small adjustments until you reach your target size.
If they're that desperate to cheat, they'll find some way, like creating a sockpuppet (assuming there aren't any post count restrictions on voting).
They do.
The easiest way to resize (as in scale) individual layers is to copy it to another document, resize that and transfer it back. Or just use the move selected pixels tool.
File size is not the same as quality. The quality slider determines how much quality will be sacrificed in this save. Any quality you have lost before is gone forever and you can't even keep the benefit of a smaller file size.
Once you open a JPEG image it's converted into a bitmap and any quality loss is similar to quality loss resulting from a scanned photo being torn. It may be a lower-quality image to anyone that sees it (just as it would be a low-quality image if it was taken with a shaky camera), but that doesn't mean it has a smaller file size.
If you open an 80% quality image, add a drawing on a new layer and save it as a 95% quality JPEG the file size will be larger and the drawing will be only slightly distorted, but the old image won't get any better.
From your blog's comments:
Would it be possible to have Paint.NET silently use the localised submenu (IE SubMenuNames) when a plugin passes something like "distort?"