As to being a mortal sin, the issue comes in of weighing the damage done against the individual (here, a large corporation); moral law sees a difference between taking without permission for someone who has little, and from someone who has great.
Further, there are still individuals out there who have no perception whatsoever that duplication of software may have any moral implications. No understanding of the moral dimension of duplicating equates with no sin. Once the awareness of the wrongness of the act, then the duplicator may have some moral issues, but people who duplicate for a friend are not doing so to the level of a mortal sin.
While you’re right that this is a diversion from the original question, which I think has been answered fully already, I really must take issue with your statement that the gravity of theft is related to whether the person stolen from has a lot or a little.
The Catechism states that grave matter consists in the moral law and that law is such that human reason alone is enough to perceive it.
While theft may be the only possibility
in extremis - that is to say when a starving man sees food that he cannot afford he may purloin enough of it to meet his needs since in the cases of ultimate need all necessary goods may be deemed to be held in common - nobody could say the copying of a computer program is normally intrinsically linked to the protection of the life of a person. Since legitimate possession of commercial computer programs or most music would normally require payment in order to do so, it doesn’t need a genius to work out that then copying to allow someone else to possess that music or computer program is wrong, therefore your example is wholly incorrect.
You are right in saying that there are people out there who claim to not realise that copying something is wrong, but that doesn’t mean they’re free of guilt. It just means they’re lazy in their thinking or making excuses because, as I’ve stated above, it should be completely obvious to anyone that looks that it IS wrong to do such a thing.
Copying something for a friend is wrong. It’s theft on the part of the friend who uses the product of someone else’s labour without adequate recompense as well as theft (or accomplice to theft) in respect of the party which copies. Whether or not the originator of the product is a one-mand-band or a huge corporation, one still derives illicit benefit for something that, in the case of software, may have cost several hundreds of dollars to obtain legitimately. And that means the theft is in the order of that same amount of money. Microsoft Office may cost, say, $400. I think that any priest would consider the theft of $400 a considerably grave sin. It is NOT a trivial matter.