One very important thing to understand is that you can't simply legislate the resultant PDF file size when creating PDF from original content.
Image size and resolution certainly can have a major effect on the resultant PDF file size, but there are other possiblities including large amounts of vector artwork for which the only way to bring down the file size is to turn it into raster image (often making it unreadable) or by simplying it.
When you refer to your EPS images (we strongly recommend against using EPS for images in the 21st century), we assume you mean raster images that were saved as EPS in Photoshop. It is possible that those high resolution images were already compressed quite a bit and thus, smaller JPEG versions of the EPS files would not have any appreciable effect on the output PDF file size.
Quite frankly, if much of your 24 page catalog is imagery, 11.5MB of PDF is not bad at all. Maybe if you can post a copy of the file, we can give you some further advise. But in the end, you might have to dramatically simplify your design to reach a target output file size.
In Acrobat if you go to File>Save As>PDF Optimised
in the Top Right Hand Corner there's an "Audit Space Usage"
Here I can see a PDF I have has 38% of it is Images and 36% is Document Overheads
See this link for some tips http://indesignsecrets.com/document-overhead-in-indesigns-pdf-can-be-huuuuuuge.php
Great feedback thank you. I'll never understand the science behind image file types but do recognise they can create different outcomes in publishing. The eps images are provided by the manufacturer of the products we sell. I just had a look at some in isolation. The clarity and resolution is excellent; some images are in excess of 1Mb, and there may be 30+ images dotted throughout my catalogue.
As for vector artwork, I believe this to be 'man-made' imagery of which there's none in the catalogue. There are a lot of tables though and a few coloured blocks.....if you are a Dropbox user, you can see the catalogue here.
More than half of your document size is "content streams" which, as I undertand things, is text and vector art, neither of which you can do much to compress. You can try replacing the vectors in ID with raster versions of the same art, if you can live with a loss of crispness.
Your next biggest chunk is Document Overhead. You can get rid of a lot of that using the methods in Eugene's link, but the bottom line is this file is probably never going to hit your 5mb target without a drastic loss in quality, if at all.