I had a problem earlier with this earlier (see
this thread) which turned out to be due to not having checked "Disable font size rescaling" under Look & feel > Fonts.
Since then I have made this the default, and I spent three days reconverting all my EPUBs with "Use saved conversion settings" unchecked, to make sure that I don't get hit by a book which uses some old settings with font size rescaling enabled.
However, I now have another font size rescaling issue. One of my EPUBs (which looks perfectly normal) has <body> tags with no associated CSS. Looking at it with the Calibre editor, the "computed final style" text font size is 18pt (the usual default 1em size), but the converted AZW3 has <body class="calibre">, and the generated "calibre" CSS class contains a line which says "font-size: 12pt". On my Kindle, this is unreadably small, and I have to zoom in to size 11 (on a scale of 1 to 14) before it looks normal, which is size 4 for any other book.
I've looked at all the conversion settings and have tried various things to get rid of this (e.g. specifying a CSS rule for <body> with font-size: 1em in the EPUB), but it keeps insisting on generating an AZW3 with a 12pt font size, and the only way I can get rid of it is to manually edit the AZW3 after converting.
I've only found this one book with this problem so far, but I have no idea how many others might be affected...
Has anyone got any idea why this might be happening?