I made the few changes announced:
if you use the site resize, it does not check file size (as it is already properly compressed)
else, it checks what size would be the picture if recompressed by the site (80% quality, interlaced) and accepts your picture as long as it is not more than 20% bigger than what it would have been if compressed by the site (and 20% is already much) no matter what was the size limit computed with width & height.