Think of standing something next to a grown person to see how big it is. The person is the ruler — and once a person is your ruler, you can measure how big a thing is, how much food is enough, and how hard a job feels.
From the picture
拿男人當「大」的標準
尺還是那個人，但量的東西變了：不再是物體多高，而是份量夠不夠多、事情有多吃力。
adjective · B2
Big — bigger than the normal size — and sold or served as the size that is treated as right for a grown man. It is used most about servings of food and about products that come in a larger size.
大份的
adjective · B2
About as big as a grown person — used to say how tall or wide something is by holding it up against a human body.
跟人一樣大的