normally in desktop applications we see that the patch only applies to the part of the application that were updated, but for android applications we need to reinstall the whole package, i mean we need to download the whole installer again. Are there any reasons (security) for this?
The installation package, the
.apk file, is signed as one entity. Android makes sure that either everything is ok or it will not install it.
But, you can split your application into different parts and update them separately since Android offers a way to let applications that share the same certificate interact in more ways than applications that do not share certificate.
Read more here about signing your application and your options: http://developer.android.com/tools/publishing/app-signing.html
In fact modern Google play will just downloads the needed parts, not the whole package. If you update usually you'll realize that it reports the size as many MBs but after downloading just a few hundred KB the installation is started immediately
Google Play has begun to introduce "delta", partial updates, the download only of the new code, reducing data traffic on smartphones.
Google Play delta updates allow to the users to save data traffic included in their subscription or simply shorten the time to download applications.
The Instagram update to the version 3.0, released in the Google Play store Thursday, August 16, 2012, included a packet of only 3MB in size instead of full application of 13MB.