JRuby is not a 1:1 mapping of a Ruby to Java bytecode -- there's significant additional book-keeping that must be done by JRuby's runtime (for example, maintaining the Ruby call frames).
In contrast, Scala maps to the JVM as closely as possible. Scala classes are Java classes -- Scala and Java are bidirectionally interoperable, and Scala's performance subsequently benefits.
In contrast, Scala maps to the JVM as closely as possible. Scala classes are Java classes -- Scala and Java are bidirectionally interoperable, and Scala's performance subsequently benefits.