The quality is not directly proportionate to the size. JPEG compression does not necessarily reduce the file size by exactly the inverse proportion. The actual reduction depends on several factors, such as:
Image complexity: A highly detailed image may still have a large file size even with reduced quality, while a simpler image might see more significant reductions.
File format: JPEG compression works by reducing quality in ways that are less perceptible to the human eye, but it’s not linear. A drop from 100% to 80% may not correspond to a 20% size reduction, it could be more or less.
Initial size and quality: If the image is already optimized or has lower quality, further reductions might have less of an effect.
Ultimately, I recommend testing to see what renders best for your site's uploaded images.