What really does a big deal mean?

Why do people say “big deal” when what they really mean is “no big deal”?

I think I get it now.
There’s a certain work habit I keep showing that drives me crazy. When I’m doing a translation project: it’s my tendency to look at bits I have already written in my translation product again, and again, and again… even though I know it’s correct. Actually, if the original version of something I’m translating is in Word format, I just duplicate it and write my English version over the French / German in the duplicate. With that measure, the formatting is practically guaranteed to be correct! But there are times when I’ve left individual words of the original in the translated product, amid the English words of the translation. And I make an effort to check for typos as I’m writing, check each sentence or paragraph for errors as soon as it has been written, but it doesn’t always work.

Having thought more deeply (if only to curb these kinds of quality lapses) I remember once reading in a job hunting guide that it stated that a common question was, “Why haven’t you found a job yet?” This is an example of how I consider that, sometimes, the view you take of a written message really does reveal who you are to a certain extent. When I first saw that question I thought of being in a job interview with an overbearing and stern interviewer, the kind of person who’s quick to be suspicious of me being inadequate but when they do it they jump in at the deep end without knowing the full story. But today I think it’s possible that someone could read the same question and think of being in a job interview with an interviewer who’s more friendly and curious i.e. “Why HAVEN’T you got a job yet?” – maybe they mean to imply despite your experience or qualifications or whatever. But my point is than when I write a translation of something, I don’t want the reader to react to anything in it in a way that’s “bad”, or a way that I (or they!) wouldn’t understand and it may well be that they wouldn’t be able to explain it all to me coherently enough.
I suppose “that’s life”.