What I would do is edit the epub after the Calibre conversion. Edit it right inside Calibre using the Tweak ePub feature.
Once you've "exploded" the ePub with Calibre, use Notepad++, an excellent free utility, to edit the stylesheet.css. To determine what style governs the bullet list, first open the specific html file that contains the bullet list and look for the style preceding the text in the list. Then look for that style in the css sheet and make the value for the "font-size" smaller. For example, if the current font-size for that style is something like 0.7057em, make it 0.5em. You will have to experiment on the amount to reduce the value to get it the way you want it, but a reduction of 0.2em is a good place to start. After you're done, save the change to the stylesheet and "rebuild" the epub.
Assuming a single style is governing every bullet list in your epub, then you should be able to make a size change to them all with just a single change to one value in the css sheet.
Make a backup copy of your epub before you start tweaking in case something goes wrong.