Sips: resize image's smallest side, not largest side
Apparently using ImageMagick yields more favorable results when compared to sips, so using ImageMagick instead might be better. The command to do so is:
convert image.jpg -resize '2000x2000^&gt;' image-resized.jpg

The 2000x2000^ resizes the image so that its shortest side is at least 2000 pixels. So, for example:
6000x4000 → 3000x2000
4000x6000 → 2000x3000

The &gt; character prevents it from acting upon images that would be upscaled, and as a result:
600x400 → 600x400
8000x1500 → 8000x1500