Originally Posted by Granny Luddite
In the conversion options, under Look And Feel, select the "filter style information" tab. Check the "Fonts" box then in the "Other CSS Properties" add "font-size" (Other CSS Properties: font-size, line-height).
Great, thanks! That fixed the problem. It is heavy-handed as you said, (titles become the same size as the text), but line spacing is now adjustable in the same range as for other books. I'll try to fix up the titles by changing some font sizes in the .css file with calibre's "tweak book".
@DNSB: I can't adjust the line spacing in your copy of the book, I tried removing the line-height terms from the .css file but then the double-spacing problem returns.
@EldRick: You are right, I had already changed the line spacing in the book.