Xcode 11.4 - Why my application size is increased from 8 MB to 80 MB with just iOS 12.0 support

I just add changed the target from 13.4 - 12.0 and my app size is size from 8 MB to 80 MB.

Can I reduce the size again with iOS 12 support?

The most likely reason is that your app uses Swift and the IPA needs to include the Swift framework for old OS versions.

But, not to worry, your IPA contains everything that any device could possibly need, but the App Store makes thinned versions so that users only download what they need.

To get an idea of what that would be, upload your IPA to AppStoreConnect and look in the "Activity" section of your app. Choose the build, and then look under "Compressed File Size" and click "App Store File Sizes" which will give you download and install size.