how do I reduce the size of a photo from 388KB to 200Kb on a MacPro?

Since you are posting in the Photos subforum, I assume that you use the new Photos app to edit your pictures.

Once your picture is in the Photos Library, the best way to reduce its size is to export it at a specified JPEG Quality (you can choose between Maximum/High/Medium/Low), size (choose between Full Size/Large/Medium/Small or Custom. In my illustration I have used Custom and set the dimensions to 1772 pixels longest side. Of course, you must fill in your specifications; for instance, on our Photography Board, maximum allowed size is 1024px longest side.

I would also suggest switching "Include: Title, Keywords, and Description" off (it's switched on in my illustration).

Sometimes (especially if you have to keep the dimensions of the photo rather high) you still cannot get the file size low enough. In that case, I delete the exif-data. As I do not know whether this can be done in Photos (I mostly use other programs including Lightroom, Photoshop... for serious photo editing), I often make use of a program called "SmallImage". I got it free from Macupdate, but although it still functions perfectly under Yosemite, it looks like it has been discontinued.

You can try use the app "Preview". Open the JPEG file with Preview, the go to "File" and choose "Export". On the bottom of the window you can choose file format and quality.

Hope this helps!