App Slicing Thinning: Pre determine actual true size & space taken by App install or update on iOS Device? Note: This has to do with App Slicing and App Thinning that was introduced by Apple in late 2015. Nothing to do with your 2012 question.  

https://developer.apple.com/library/ios/documentation/IDEs/Conceptual/AppDistributionGuide/AppThinning/AppThinning.html

http://www.imore.com/app-thinning-ios-9-explained

http://www.macrumors.com/2015/10/05/ios-9-app-slicing-now-available/

http://www.appcoda.com/app-thinning/

Given that my iOS storage has been running out of space, I try to keep iOS App versions which are smaller in size and not update unless essential or compelled to {e.g. Uber compels updates} 

So, I recently did a comparative analysis for 2 apps and what sizes (in MB) they show in 3 places (iTunes 12.3.3 On Windows 8.1 x 64): 

App Names:
Todoist  11.0.3
Wunderlist 3.4.3



Current Latest Versions updated &amp; synced this weekend: 

iTunes > AppStore | As shown on AppStore before download
70
95  

iTunes > My Apps  | As shown in iTunes after download
34
53  

iPad 2 - iOS 9.0.2 > | As seen under iTunes > Sync > App Selection area after Install/ Sync
65
87  

iPhone 5 - iOS 9.0.2 > | As seen under iTunes > Sync > App Selection area after Install/ Sync
XX
YY
{Will post these numbers soon as well}

I cannot understand why the "complete un-thinned App from the AppStore would take more space when thinned for the iPad or iPhone"
I can see multiple reasons for that happening:

A) Compression

Apps are downloaded in compressed form from the app store. It is similar to a Zip-file. When the app is installed on the phone, it is uncompressed by the app installer software on the phone.

B) Differential updates

If you already have the app installed, and are merely downloading an update for the app - you could be downloading a differential instead of the full app. For newer iOS versions, the App Store will calculate the difference between what you already have and the new version of the app, enabling you to download less. However it will still take up the full amount of space on the disk after the upgrade has taken place.

C) App downloads after first use

When you use an app for the first time, many apps will download extra data that is stored with the app on your phone. This could account for differences in app size as listed by the App Store, compared to the amount of space the app takes on your phone in practice.