Thanks - I do that, and click everything "off" but the file is still big, hence my question about whether there is something included that I can't avoid, that pumps up the size.
I guess I could make some experiments to see if a given file size in C1 looks the same at the same size exported from PS ...
It's not only the amount of compression, but also the amount of chroma sub-sampling
that changes in Photoshop. Perhaps C1 always uses a better quality sub-sampling, even when compression is set to high (overall quality is lower)?
I haven't tested the C1 JPEG quality, but that is where you can focus your attention to spot the differences. You'll notice the increased loss of fine color detail when the compression increases. You can e.g. use this image
from Roger N. Clark to test (convert from indexed to RGB mode first, and save as a TIFF for use with C1). The compression/subsampling artifacts look different at different brightness levels, so you could make a brighter and darker version as well. There can also be a difference between the accuracy of horizontal and vertical features.
You can also use the image used on this page
, and count the number of remaining colors.