I can believe it if the application is written properly using a functional paradigm to make it parallelizable. In Java this almost inevitably means making endless copies of objects to pass to functions and then immediately discard, except the discard isn't immediate. It doesn't happen until the garbage collector kicks in. So you end up with a memory use graph that looks like a sawtooth as the garbage collector is discarding hundreds of megabytes of temporary objects every time it runs.

I've seen this on a modded Minecraft server with a half dozen players connected. It would thrash over a gigabyte of memory every 5-10 seconds or so. I had to tweak the garbage collector to avoid performance hitches.