Proguard

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If you have some restrictions on your JAR size (for example when deploying a free application to heroku), you can use proguard to shrink it. If you are using gradle, it is pretty straightforward to use the proguard-gradle plugin. You only have to remember to keep: your main module method, the EngineMain class, and the Kotlin reflect classes. You can then fine-tune it as required:

build.gradle

build.gradle

buildscript {
    ext.proguard_version = '6.0.1'
    dependencies {
        classpath "net.sf.proguard:proguard-gradle:$proguard_version"
    }
}

task minimizedJar(type: proguard.gradle.ProGuardTask, dependsOn: shadowJar) {
    injars "build/libs/my-application.jar"
    outjars "build/libs/my-application.min.jar"
    libraryjars System.properties.'java.home' + "/lib/rt.jar"
    printmapping "build/libs/my-application.map"
    ignorewarnings
    dontobfuscate
    dontoptimize
    dontwarn

    def keepClasses = [
            'io.ktor.server.netty.EngineMain', // The EngineMain you use, netty in this case.
            'kotlin.reflect.jvm.internal.**',
            'io.ktor.samples.hello.HelloApplicationKt', // The class containing your module defined in the application.conf
            'kotlin.text.RegexOption'
    ]

    for (keepClass in keepClasses) {
        keep access: 'public', name: keepClass, {
            method access: 'public'
            method access: 'private'
        }
    }
}

You have a full example on: https://github.com/ktorio/ktor-samples/tree/master/generic/samples/proguard