OTA update file size

 If you are using smart phone from last some years Then you notice that OTA Update file size of Nokia symbian , android and windowsphone devices. are small . Basically OTA updates system is very common in every OS...
            but in Xiaomi devices its kind of difference..
         In xiaomi instead of just updated file whole ROM is dowloaded and whole ROM is flashed however in other devices its just necessory file only update. and it also less in size.. and hence it looks like very big in size..  however actual update is very less