When we compiled Hello World! and produced the JAR, we instructed the compiler to bundle in the Kotlin runtime. Why is the runtime needed? Take a closer look at the bytecode generated, if you haven't already. To be more specific, look at line 3. It invokes a method to validate that the args variable is not null; thus, if you compile the code without asking for the runtime to be bundled in and try to run it, you will get an exception.

$ kotlinc HelloWorld.kt -d HelloWorld.jar
$ java -jar HelloWorld.jar
Exception in thread "main" java.lang.NoClassDefFoundError:  kotlin/jvm/internal/Intrinsics
at HelloWorldKt.main(HelloWorld.kt)
Caused by: java.lang.ClassNotFoundException:  kotlin.jvm.internal.Intrinsics

The runtime footprint is very small; with ~800 K one can't argue otherwise. Kotlin comes with its own standard class library (Kotlin runtime), which is different from the Java library. As a result, you need to merge it into the resulting JAR, or provide it in the classpath:

$ java -cp $KOTLIN_HOME/lib/kotlin-runtime.jar:HelloWorld.jar  HelloWorldKt

If you develop a library for the exclusive use of other Kotlin libraries or applications, then you don't have to include the runtime. Alternatively there is a shorter path. This is done via a flag passed to the Kotlin compiler:

$kotlinc -include-runtime HelloWorld.kt -d HelloWorld