Hi, I'm really new here so I'm not sure if any of you are jailbroken or not. That's the only easy way I know to investigate this. There is a Cydia app called AppInfo that's free & amazing-it calculates the actual size of your apps. You can do this individually or export a list.
You'll probably be surprised/dismayed by what you learn. For example AppShopper, one of my most-used apps, was racking up a cache of 1.5 GIGABYTES after being on my device for over 6 months. I had to start periodically deleting & reinstalling it because it grew at about 300Mb per month. Cinetap for Netflix is another one I've caught creeping up past the 500 Mb mark. Most RSS readers, if you have a lot of feed subscriptions, may also grow to epic proportions depending on whether or not you're telling them to download images/full posts. I wrote to the AppShopper devs a while ago asking about the cache issue and never heard back, but lo! Their last update mentioned "vastly improved cache handling" and sure enough I had over 100 extra Mb after updating.
I think a lot of it has to do with poorly handled image caching. Without AppInfo or something similar I'm not sure how you'd determine the worst offenders, maybe someone else will have a tip but you cannot see it in iTunes, only the original download size. Which in my repeated experience can easily be 100mb-1Gb less than the space it's actually using. Hope this helps.