When you start your java app you pre-allocate all the memory you're going to need for the process' lifetime. This could be 512MB, this could be 2GB+, but it's never small. And this is unaffected by your initial traffic volume. It's the same for 0.001 hits/sec or 5 hits/sec.
On the other hand, if you drop a bunch of PHP scripts on a server, Apache only uses the memory it needs. Sure, you may have some pain scaling it later, but to get up and running you can get a cheap server and run your code.
Also, while I have your attention, have you ever tried installing an enterprise java webapp? It's not like you can pile a bunch of them into one JVM on a machine. I run confluence/jira/bamboo. Each has it's own requirements and basically need to run in their own tomcat servers tuned with their own settings. It's a huge PITA and eats up a ton of memory.