The Tips in detail

 "Forget about the Memory-Usage-Information in the rendering progress window."

I don't know what information 3DSM2.5+ gathers here, but it is not what your system actually uses. If you dare to know how much memory is used you should refer to the task-manager (provided you are using NT) and have a look at the systemperformance-tray. The informations provided there are much more reliable. Consider the numbers in the rendering dialog a rough estimation.

Estimating the size of a scene depends on various aspects. The materials used, especially any bitmaps, increase the size. The more vertices and polygons are used in any scene, the more data needs to be stored. The actual size depends on applied modifiers and animations as well.