You can better manage Tomcat memory usage by creating a SETENV file defining JAVA_OPTS environment variable settings in the tomcat/bin directory.
Mac/Unix: create a file called setenv.sh and add the following line:
export JAVA_OPTS="-server -XX:+UseParallelGC -Xmx768m -XX:MaxPermSize=160m -Djava.awt.headless=true"
Windows: create a file called setenv.bat and add the following line:
set JAVA_OPTS=-server -XX:+UseParallelGC -Xmx768m -XX:MaxPermSize=160m -Djava.awt.headless=true