Yes, it could be a bug. But it's possible that they did something stupid when creating the ePub like specifying a fixed point size and the reader is obeying it. It's worth reporting, I think - in the former case, they can fix it, and the latter might get Kobo to add in some override options for fonts.
(That, and margin justification. There's no excuse for an ePub to not have the margins justified, but that hasn't stopped Simon & Schuster...)
|