LR4 and making a pdf book
I think you will find that the PDF's are large because the assumption is that you would want to have the book published/printed so that the quality is satisfactory.
In fact it is a PDF that LR sends to blurb.
I made a 13 ' X 11" book of 120 pages and the PDF size was enormous and took some time to upload to Blurb.
My only suggestion is to use LR to produce the PDF then use a PDF utility program to down size it.
eg use something like nitro PDF creator to print to a smaller PDF file.
The quality for viewing on a PC screen is no where near the quality required for a high quality book.
Hope this helps