I need to reduce the size of a epub for use on my andriod phone

[#1]

Hi
Help please.
I have an epub which is 4Mb and is a collection of 5 books. It take 15 mins to load on my Android 2.1 phone
Ideally I would like to split each book into a new epub which would load faster 
I have downloaded an epub editor called 'calibre' but cannot see how to do this 
Steve

[#2]

epub is essentially a compressed set of HTML files  containing the book chapters/parts. You can expand/recompress the files using standard windows tools.
In essence the process is something like this ....
load the file onto a windows PC;
make a copy
rename the copy with a .zip filename extension
locate the renamed file using File Manager
rightclick on the zip file name
Windows should be able to use the expand option to create a expanded subdirectory containing each of the component files
delete some of the (html) files in the expanded directory
recompress (right click in file manager and then send to compressed/zip format)
rename back to something.epub
reload on android

[#3]

Thank you  AlaricAdair a great insight into the workings of an epub
An excellent reply. I can use this method to adjust the file size on a number of short story compliations i'm having trouble reading on my phone.
Steve

[#4]

That's helped me a load too, thanks AlaricAdair - I've had several that aren't displaying on my phone correctly and will now be able to go and tinker without having to wade through calibre (nice as it is, it's also very cumbersome).

[#5]

It comes from hard experience of helping my publisher debug the epub versions of my books. Somehow Microsoft Word inserts duff characters in the doc files which get fed into Adobe InDesign for publishing. The duff characters don't show in the pdf version of the book, they even print correctly at the print house. It was just when we were generating the e-book versions that we'd see spurious character appearing in the text body of the epub file. Eventually after a lot of work we found the only way to eliminate the spurious characters was to crack open the xhtml files from the epub. We'd then use Dreamweaver to edit the XHTML to remove the duff characters and then repackage the XHTML files into the epub format.
I now use LibreOffice Writer when I'm writing rather than MS Word. It didn't matter how often we stripped out all formatting of  text using Microsoft Word the spurious "hidden" characters. We suspect it is some corruption of the database hidden within the Doc file. It is actually quite an interesting study to find what MS hide in their Word Doc files.