[GeoffR]

I don't know about the problem with KoReader, but the reason the font size on the dropcap is different in the Kobo ePub and KePub reader is probably because the book specifies the font-size in px or an absolute unit such as pt. This doesn't work the way you might expect in the ePub reader, and I think the only way to solve it is to edit the stylesheet so that all font sizes are in font-relative units such as em or %, which should work the same on all devices.

Edit: That doesn't just apply to dropcaps, but to all font sizes and other sizes in the book. It is impossible to get anything looking the same on devices with different screen sizes, resolutions and pixel densities if sizes are specified in pixels or absolute lengths. This is a common problem with older ebooks where the publisher seems to have assumed that ereaders would never change, and a lot of books appear to have been designed on the assumption that all devices have a 800x600 6 inch screen.