**I can only guess the meaning like : "What do you mean by mentions N here ?"
but I'm not sure Hope someone can help me understand its correct meaning.**

Context: MC is trying to learn cooking in order to reopen a restaurant for a
friend (he's also worked there before, but only as a waiter) .And then 1 of
his other friend (A) asked him about this.

A「そういえば、Bはお店を再開しようとしているのであろう」

B (MC)「あ、うん……そうなんだ」

A「いつだ。いつからだ。大盛りか」

**MC「何が大盛りなんですかっ」**

MC「Mさんがほとんど仕切ってたけど、その仕事を今調べてて……中々大変です」(M-san is the previous manager of the
restaurant, she did most of the cooking parts for the restaurant. But now
she's gone, and MC is trying to learn from a notebook that M-san had left
behind.)
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2022-11-19T04:41:08.610
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2022-11-21T16:33:33.553
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question
grammar meaning expressions japanese-to-english
The meaning of (nani ga) 何が + N (something) + なんですか (nan desuka) in this context
Presumably it points to the fact that there is something absurd about the
previous utterance, particularly around 大盛り. However, what is absurd is
difficult to tell from the given context (and frankly, I think that's more of
a story comprehension issue than a language issue, so I'm not sure if
additional context will be helpful here without going off-topic.)
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2022-11-19T06:12:26.457
97285
2022-11-19T06:12:26.457
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answer
It's similar to something like:

A: Hey, this photograph looks beautiful.  
B: It isn't really "beautiful". (or B: What are you talking about.)

It's not necessarily (although it could be, just not in this case) a sarcastic
way of reacting to a compliment or statement, but rather them reacting in a
way like A does not make sense by calling the photograph beautiful.

In your example's case, A asking about whether it's going to be very
beneficial (I'm assuming financially, making B wealthy) makes B react by
questioning what A is saying; as if B never had that aspect in mind when
making the decision because it would turn out to not be all that beneficial.
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2022-11-21T16:33:33.553
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answer