The JZopfli Maven plugin processes all artifacts and improves the compression using the Zopfli algorithm. In addition it removes useless metadata inside the zip archive to improve further the file size. See the details here.

Simply add this to your pom.xml

<build>
    <plugins>
        <plugin>
            <groupId>com.github.luccappellaro</groupId>
            <artifactId>jzopfli-mvn</artifactId>
            <version>0.0.4</version>
            <executions>
                <execution>
                    <goals>
                        <goal>jzopfli</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>