Okay, plug-ins to the rescue. I've loaded
Jeffrey Friedl's "Metadata Wrangler" into Lightroom and all is well. A 150x200 jpeg pixel portrait of 80 quality with the plug-in on full blast (all metadata stripped) makes a 12 KB file. Without use of the plug-in, the resulting file comes in at 40 KB. That's a big difference.
If I set the plug-in to preserve just the copyright and caption information, the resulting file still takes up only 12 KB.
One interesting note. Lightroom has a check box by which you can "Limit File Size To:..." and the program will give you the highest quality for a selected file size. But I think Wrangler does its thing after the jpeg is generated, so the "Limit..." function might not work, and you'll be told a jpeg of a certain small size is not possible. For example, if your original contains 30KB of metadata, Lightroom won't start to make you a Jpeg of 20 KB.
If you're still with me, I think I have a workaround:
1-Make a jpeg with
no metadata.
2-Without changing quality or size settings, make another jpeg from the same original with
all metadata. Don't overwrite.
3-Figure out the difference in the sizes of the two resulting files. That's all metadata.
4-Add that metadata amount to the "Limit..." setting you really want, and switch on Wrangler.
The result should make you happy.
Unless it contains pictures from vastly varying times and processing, I think you can assume that all the pictures within a folder will contain about the same amount of metadata.
MB