If I look at the usage of the memory of the iPad I see that half of the capacity is being used by apps. Is there a possibility to see details which app uses how much memory?
A couple of points. If you're used to a conventional PC you may find Apple's eccentric use of the term "memory" to be confusing. The original iPad has 256K "memory" (meaning RAM) and the iPad 2 has 512K RAM. Apple doesn't publicize these numbers, probably because they're paltry compared to most conventional computers (and some of the iPad's competitors.) What Apple calls "memory" is actually a combination of the RAM an app requires and the storage associated with the data it accesses.
To understand this in conventional terms, assume that you have 15 gigabytes of MS Word docs on a PC and that the Word application itself is (just for the sake of illustration) 800mg in size. In Apple's view, that means Word would be 15.8 gigs in size. And if you added another five gigs of documents the Word app would be 20.8 gigs.
Furthermore, if you had, say, 20 gigabytes of jpg files on your computer with multiple apps designed to display jpg's, you wouldn't say that each app is 20+ gigabytes in size. But because of Apple's OS architecture, that would be the case with separate storage required for each app that accesses a particular jpg.
What this means is that if you subscribe to a magazine, such as the New Yorker, the app will consume more "memory" as you download additional issues of the magazine. That, of course, is an odd way to think about it. Much like saying that if you add music or video content to your iPad, the apps that access that content "grow" in size. My New Yorker app, for example, now consumes about 3Gigs of "memory." Each time I add another issue, it grows, despite the fact that the app itself has not changed. And if I "archive" an issue (i.e. remove it from my iPad), the New Yorker app will "shrink" in size.
There are reasons other than marketing for Apple's approach. For one thing it reduces the chance that a virus embedded in data (i.e. content) can "leak" into a common area where it can affect the overall performance of the iPad. But as noted above, if you have a single PDF and two apps to display/modify it, you'll have two copies of the pdf and changes made by one app won't be reflected by the other. Further, it makes comparing the size of apps difficult. A PDF reader associated with one document, for example, will be much smaller than an identical app with 50 pdf's associated with it even if the "apps" are exactly the same size.