What they have in common is OpenGL. You're using the preview renderer, and probably using hardware accelerated preview. As the features and abilities of OpenGL are increasing with each release, so too does Poser's reliance on those features. It's not unusual that P6 isn't revealing a problem while P7 and PPro 2010 are.

You are rendering by the way, in the sense that the scene data is being made into a series of images. Those images don't exist if you don't render. Any data hanging around between one image and  the next (in the renderer) can cause memory to be used up. When it isn't released, that's called a memory leak.

It is possible that Poser is leaking, but more likely it's the graphics driver. Since you say it suddenly started doing this in two versions of Poser, it's likely that you suddenly updated to a buggy video driver. Windows is notorious for doing this in the background as part of Windows Update. Sometimes it will actually mistakenly send you back to an earlier version.

If you're going to use hardware preview rendering, you really need to pay attention to your video software version, since it has become an intergral part of preview rendering.

You might also try SreeD preview mode (all software) to help verify that it isn't actually Poser that is leaking, but the hardware driver.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)