The article doesn't say what the -Xms was. If you set it to less than Xmx, the VM will increase the memory size only when it's really necessary - it will issue

The article doesn't say what the -Xms was. If you set it to less than Xmx, the VM will increase the memory size only when it's really necessary - it will issue a GC before trying to increase it. A GC on a heap of 100M objects takes a while. Even when you reach the Xmx the VM will try to GC, and sometimes it can free a bit of memory so it doesn't throw an OOM, insert another 10 items and run the GC again, and so on - as long as you allocate small objects, it can take a while until you see the OOM.