BBeB is based on Xylog XML, which is then compiled into a binary form.
Why anyone would think "XML" would make a smaller eBook format is baffling to me. What makes a smaller eBook format is being able to compress the text streams inside the eBook.
BBeB is handicapped in this department as it only allows sentence level compression via zlib compression in it's page layout mode. The compression dictionary is built on each string so it never gets very efficient.
I had hoped Sony might implement a document level compression dictionary, which would greatly enhance the compression in BBeB, but alas the format is apparently compact enough for Sony.
