I agree with Pavel Rozenblioum in that the Java example is a bit inflated. Please consider using https://github.com/yunspace/lambda-platform-perf-comparison/pull/1 as a better apples <--> apples “Hello” example to the other languages. My jar size produced was 10k now.
Your Java code is very different from the other . In Java your response serialises the String into JSON using a very powerful library while in the other languages you just set the String. Removing that step should make the execution faster. Removing the Jackson dependency will make the code size smaller. You can probably remove the log4j dependency as well making the code even smaller.