Emmanuel,

Thanks for your explanation.

When I Save & Compress, the "dirty" file becomes 201 Kb (instead of 595) and just the helical staircase shrinks from 468 Kb to 124 Kb.

For a staircase a growing file isn't a big problem, most homes only have one. But for a repeating model (chair or table in a restaurant?) it would make a big difference.

Now I know what the reason is, I hope I can sleep well

Hans