I've just installed Tomcat 6.0.26 as a test. It worths note that its server is launched within a file called tomcat6.exe instead of java.exe (when you need to find it with task manager).
Here's the table summarizing the differences. Hope this could
give you some help:
AD server NT service
commit charge: 450MB
java.exe: 153MB
AD server "bare"
commit charge: 84MB
java.exe: 35MB
Tomcat 6.0.26
commit charge: 44MB
tomcat6.exe: 29MB
