One of the major themes of Ecclesiastes is vanity, meaning that all we do in life is in vain, empty, and useless. No matter what we do with our lives, it means nothing to us at death. We are advised to enjoy life when we can, but we are given examples of what happens when we forsake that in the pursuit of wealth and fame.
All is vanity and a chase after the wind
2:17 NA "Therefore I loathed life, since for me the work that is done under the sun is evil: for all is vanity and a chase after the wind."
I selected the mouse running on the wheel as the ultimate symbol of wasted activity. Always running but never getting anywhere. As productive as chasing the wind.
I undertook great works
2:4 NA "I undertook great works: I built myself houses and planted vineyards."
When it comes to physical structures, perhaps nothing will ever top Ancient Egypt for excessive personal monuments. I guess they were partly successful in that we remember many of the individual pharaohs today, but considering the cost and labor involved, one can imagine what might have be created for the living instead.
4:8 NA "A solitary man with no companion; with neither son nor brother. Yet there is no end to all his toil and riches do not satisfy his greed. 'For whom do I toil and deprive myself of good things?' This is also vanity and a worthless task."
The large scale of the well appointed bedroom and the small single figure within were meant to demonstrate the idea of someone whose pursuit of wealth came at the expense of having no one to enjoy it with.