Huge ePub file size

Hello

Hopefully someone is able to explain or help here?

Recently just created a big Pages book, 440 pages long with lots of images on each page. It was set up as a fixed format and the Pages file ended up being a little over 400 MB in size. When exported as an ePub it became around 600 MB.

As all the images were originally imported as PNG files I decided to re save them all adding more compression and using JPEG instead. I managed to get the Pages file down to a staggering 90 MB in size while still retaining good image quality. However, when exported as an ePub the new file was over 800 MB in size?

I then used the Reduce file size option from within Pages which then made the Pages file around 60 MB in size. However, when this was then exported as an ePub the new file was over 1.1 GB in size?

Does anyone know why this is happening. The smaller the Pages file is, the bigger the ePub file has become?

Many thanks.