I'd really suggest getting rid of all the kibble (there is no good dry food for cats, but especially not a "diet" or "light" one). Switch to all wet food, preferably one with little to no grains and no by-products, and you'll be amazed at how much healthier (and slimmer) your cats become. Some reading on feline nutrition for you to check out:
www.catinfo.org
Particularly focus on this section about feline obesity and dry food:
http://www.catinfo.org/?link=felineobesity