Here is a photo of my sony PRS-300 displaying an ePUB eBook I bought from Sony. I used ineptepub to deDRM it and Calibre to reset the margins to 15 points on top of page and 5 points on the sides. This view is using the smallest font size. To me this is a very readable eBook once I modified the margins.

Madam Broshkina is absolutely correct in her statement of how ePUBs display better on other reader devices than they do on the Sony devices. Each reader device capable of displaying ePUB files has its own unique rendering software which translates the ePUB file into what we see on the display. some devices do a great job, while others do a bad job. Unfortunately, Sony Readers do a rather bad job of displaying ePUBs. That is not the fault of ePUB formatting, it is the fault of Sony/Adobe rendering.