I have an application for which I have enabled GC logging. The heap appears to grow then takes a sudden drop, but does not log a Full GC. If there some startup parameter that I can enable that will show me what GC event is reducing the heap size?

My environment: Linux 64-Bit, java 1.6.0_31, Java HotSpot(TM) 64-Bit Server VM (build 20.6-b01, mixed mode)

VM args: -server -Xms2560m -Xmx2560m -XX:+UseConcMarkSweepGC -XX:MaxPermSize=256m -XX:-PrintGC -XX: -PrintGCDetails -XX:-PrintGCTimeStamps -Xloggc:/xxxxx/gc.log -Dsun.rmi.dgc.client.gcInterval=86400000 -Dsun.rmi.dgc.server.gcInterval=86400000

3057.609: [GC 2397254K->2385777K(2619328K), 0.0572310 secs]
3058.898: [GC 2402801K->2391301K(2619328K), 0.0566620 secs]
3059.940: [GC 2408325K->2397156K(2619328K), 0.0534080 secs]
3059.995: [GC 2397265K(2619328K), 0.0069950 secs]
3065.635: [GC 2414180K->2404934K(2619328K), 0.0732700 secs]
3065.849: [GC 2419994K(2619328K), 0.1150630 secs]
3070.248: [GC 1593931K->1591825K(2619328K), 0.1084230 secs]
3072.440: [GC 1608552K->1606431K(2619328K), 0.0533140 secs]
3087.759: [GC 1623455K->1614544K(2619328K), 0.0215850 secs]

What event is causing the heap to shrink between the output at 3065.849 and 3070.248? Is there a VM param that will log it? I tried adding -verbose:gc but that does not change the output.