reduce large CMYK file sizes

Just wondering if anyone can explain why CMYK image sizes are so much larger than their RGB counter-parts.

I ask because I'm working with our company logo in illustrator, and CMYK offers the most accurate match - when I convert to our "RGB values", the colours appear a bit darker, slightly different hue - so, I export the .ai as .jpg and use those files where needed (e.g. A4 document headers etc.). The problem is, a CMYK jpg (same dimensions etc.) is up to 8 times the size of it's RGB equivalent - so this can make documents very large (e.g. MS Word and .pdf exports). I've found a great work around - to create a template (for other employees to use), and to 'compress' the image once in MS Word through it's surprisingly effective compression tool. However, I've also provided the CMYK .jpg's as stand-alone files for employees to use when they're working on a unique document that may not already have the header and footer. So two questions:

1) How do I reduce the file size of a CMYK file so that it still prints accurately (and without any noticable compression)

2) Would this mean that my RGB colours need to be re-defined? Appreciate an RGB print is going to come out quite differently, but theoretically on screen, the RGB values should look very close to their CMYK standard (which was the base colour that we used when building our branding)

Any and all advice, much appreciated - many thanks!Start Free Trial