[PatNY]

I frequently "compact" epubs by compressing jpegs further and optimizing PNGs. Here's the fastest way I've found to do it:

1) Explode the epub using Calibre's tweak book feature.

2) Navigate to the images folder and select all. Drag and drop them onto the interface of Fotosizer, a free batch photo resizing program.

3) Set the compression level. I usually set it to 55 but you can go lower if you want even smaller files. Adjust the resize settings if you want. I usually keep all my images at their original resolution (default setting) as I'd prefer more compressed jpegs over lower-res photos.

4) Set the destination directory -- I just always use "same as original"

5) Click "Start" and in about a minute or less, all the images you selected will be resampled and compressed.

6) Rebuild the book.

You should now have a book that is significantly smaller than the original. On a 55 setting, I can get space savings of up to 70% or more.

If the images are PNGs, I use PngGauntlet. It is also free, and you also can just drag and drop your images onto the interface and batch process them. However, since optimizing PNGs takes far longer than resampling jpegs, the process can take up to an hour or more, depending on how many PNGs are in your book. Since I don't like to leave a book "exploded" in calibre for a long time, for PNGs I will just copy the originals onto a folder on my desktop. I then optimize them from there. When it's done, I just replace the originals in the exploded book with the optimized ones. The space savings with PNGs is not as great as you can get with jpegs. Maybe 10-20%.

--Pat