Item (direct transcription):
You can’t put 6-pounds in a 5-pound bag.
The informant learned the proverb from her architecture mentor whom she worked for during her education. He would tell the proverb to his clients when they were requesting the impossible of him. In the context of architecture, the proverb means that there is only so much that can be fit into a finite amount of space, regardless of the skill or ingenuity of the architect.
The informant continues to use the proverb in the same way when consulting about architecture.
The informant says she would use the proverb when someone has unrealistic expectations for what can be fit into their house plans.
This saying meets all four of the canonical criteria for a proverb. It is (1) short, (2) fixed-phrase, (3) rhetorical, and (4) metaphorical.
This proverb is an example of occupational folklore for the occupation of architects.