There are so many threads on the ePub font problem ... I'll try this one here.
I purchased a HarperCollins ePub from Kobo. It displayed correctly on the Kobo. But, I didn't like the cover and wanted to add a few comments, so I removed the DRM. I checked the file in Kobo -- it still displayed correctly. In other words, the DRM was gone, but the file itself was not altered.
Then I added the non-DRM edition to Calibre, made my change to the cover, etc, and allowed Calibre to recreate the ePub. I fired up the file in Kobo and -- voilá -- the font was tiny.
I then went back and compared the html and the .css files in the non-DRM Kobo original vs the post-Calibre converted files.
What do you know
: all the wonderful formatting created by LD.css (size 6,214 bytes) and the original html files (such as chapter 1 at 17,419 bytes) in the Kobo edition had been overwritten by Calibre and a stylesheet.css (3,728 bytes) and newly crafted html files (such as chapter 1 at 17,924 bytes) as created by Calibre.
In other words -- while I thought Calibre was merely replacing the cover, and adding a few comments in the "overview", Calibre had done a wholesale reformatting of the entire book.
No wonder the ePub files appeared to be broken ... in this case it was the conversion itself which broke them.
Calibre is a fine tool -- but in this case it got in the way of a perfectly fine book. Obviously, where books arrive already with tiny fonts, Calibre is not to blame. So the fix the Kobo folks are working on -- to interpret CSS files in a new way -- is most welcome and may, passing, "re-fix" the fixes Calibre adds.