Data toll tripled from Android to IOS, no changes in usage habits. How to troubleshoot? Possibly redirect and sniff (tips on how)? So I've just bought my iPhone 5s with iOS 7.1.2, moving from Android. I've noticed that my data toll has more than tripled (from around 300MB to 1.1GB monthly) without significant changes in my usage habits.

Strange things like Calendar using 30MB (I can't imagine calendar events as being BLOBs), Safari almost 500MB (no youtube or anything over 3G) and Facebook 250MB (auto-streaming disabled), while on android those three combined wouldn't have costed me more than 200MB. Again, I haven't changed my usage habits significantly. System services alone costed me more than 100MB (yes, I live in Argentina, 500MB is one of the typical allowances, and no, moving to SF is not an option right now)

My first question would be, do you know of a way to troubleshoot this issue?

I'm a bit hackerly, so my first approach is trying to sniff on my traffic to determine what kind of data my iPhone is so eagerly exchanging. How would you go about driving all internet traffic through a proxy (not just HTTP)? All 3G traffic?

Is it reasonable to assume that my iPhone has the same internet activity over WiFi and over 3G? 

Is there anything else you'd advise me about this? Anyone else with the same problem?
You have multiple questions here, so one answer at the time:

When your iPhone is connected to a Wi-Fi network, any app that needs data from the Internet will use the Wi-Fi network instead of the cellular data network. The data usage is therefore not counted against your monthly cellular data limit. However, when not connected to a Wi-Fi network, any time an app needs data, it uses the cellular data network and is counted towards your monthly limit.

Stopping apps from using Cellular Internet data:

That can be done in your iPhone settings. Select Cellular and it will open new window showing who and how much. There is a slider to turn that app off from Internet data.

At the bottom of the list, there is an item named System Services. Selecting it will show how much cellular data is being used for things like Messaging Services, Push Notifications, Documents &amp; Sync, iTunes Accounts, Siri, Voicemail, Software Updates, Find My iPhone, Diagnostics and Apple ID Services.

Remember those are cumulative data, so rest them once a month in date accordance to your plan.

There are few external apps for measuring and controlling internet data.

For Safari and FireFox, go to the individual settings and disable the FlashPlayer to prevent sites loading unwonted flash based advertisements. You will be asked to enable it if you do want to watch something.