Why does the apps in iPhone take up data without and with using them?

Like I used twitter for like 20 minutes two nights ago and it went up to 80 MB and like the original MB is like 30

I found a good tip online for iphone use: Each night double click the start button and at the bottom of the screen you'll see all your open/running programs. Press and hold any one. They will start wiggling like when you delete but now you'll only see a minus in the top left corner. Go thru all of them and hit the minus, this will just close the app, not delete it. The more you close the more space you will have and the less battery usage you will have.

The app itself takes up space, and the information it stores (if any) also takes up space. So there's a minimum that each app will take up, and that can grow if it stores additional information.