The Fireworks "better quality' setting corresponds to
Quality: 80
Smoothing: No Smoothing
Sharpen color edges (checked)
Remove unused colors (checked)

If I open a 66k .jpg image I have previously saved at Quality in the high 90s,
"better quality" will result in a .jpg at 16k file. I usually export with
quality above 90; 80 seems extreme for all but the softest of images. If I was
going to save .jpgs of artwork, I wouldn't use the "better quality" setting.

What you need to do is understand what your setting are. I'm pretty sure you
can set the parameters for your batch conversion to be whatever you want.
Experiment with a few of the images you have to determine what is optimal
for your images and define a new export preset (or more than one) to
use. Images with higher contrast (either luminance or color) will need a higher
quality setting to achieve the same relative faithfulness in reproduction.