I've made a simple iPad app with 30 lines of code. No image is present and its icon size is also low, but the app size is 45 MB. I have seen an app which was released in November 2015, but its size is 13 MB. My app is just a clone of that app. Why is my app size bigger, is it because of xcode 7.1?. Should I use an older version? Is there any Xcode trick so that I can reduce size, or any third party tool to reduce app size?
You need not to worry about size of app. Basically Xcode creates a generic binary and once it's uploaded to app store, only related files (images@2x, @3x etc) will be downloaded depending on device. This is called app thinning.
The store and operating system optimise the installation of iOS, tvOS, and watchOS apps by tailoring app delivery to the capabilities of the user’s particular device, with minimal footprint. This optimisation, called app thinning, lets you create apps that use the most device features, occupy minimum disk space, and accommodate future updates that can be applied by Apple. Faster downloads and more space for other apps and content provides a better user experience. You can read more here.
Also check if you have not added using Header Search Path/ Some framework.