Think of someone putting a thing down on the table and saying, 'this one counts.' A deal is something people take seriously enough to talk about, and a big deal is one that takes up a lot of room in somebody's head. Every use of the phrase starts from that claim about size. English then plays with the claim: you can agree the thing really is that big, you can deny it, or you can say that somebody has it up.
From the picture
把份量壓下去，或把份量吹起來
這一家還是在講份量，只是說話的人不接受眼前這個份量。核心畫面裡那件被放到桌上的事，在這裡被推回去（沒那麼大），或被指出是別人硬撐大的。所以同一個詞會出現在安慰、酸人和抱怨三種完全不同的場合。
noun · B1
Used to show that you think something is not important, or not as important as another person thinks. It is said on its own as 'Big deal!', and it is used inside 'no big deal' and 'What's the big deal?'
沒什麼大不了
noun · B1
An event or situation that really matters to someone, or that feels important and exciting. It is usually used with 'a', and often with 'for' or 'to' the person it matters to.
大事
noun · B2
A person who is important or well known, especially in one field of work or in one place.
大人物