According to this, the material gaining was bigger if I moved Bxc6, as the computer suggested, but is the blunder just because of this reason or is there some positional issue that I'm missing too?
Why is this a blunder?
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There are several factors in play. For one, actually, a minor piece is worth more than an exchange (that is a win of a rook for the price of a bishop/knight) materially. A minor piece is generally worth 3 points, while a rook is 5 points. So if you play Bxc6 you get 3 points, whereas Bxa8 gives you +5 -3 = 2 points. As such winning the knight is advantageous. Something else to keep in mind are the strategic aspects. Both your bishop and his knight are active pieces, while his rook is stuck on a8 currently doing very little, if anything at all. So if you take his rook, you'll exchange your most active piece for the moment for his most inactive piece, and strategically that's not something you want to do at all. If you take his knight on the other hand, you'll retain your active bishop, while his active knight is out of the game, and you'll have a much better game as a whole.
Oh, and to add to that, rooks are generally better suited for the endgame than their minor counterparts, and as such you'll have much better control of the game during the middlegame by taking the knight, and the advantages of that will, in the long term add up to make your endgame much better, as well.
Oh okay, I understand it. I have to keep in mind this concept of active/inactive pieces, because I decided to move Bxa8 just because I was thinking that in the long term that rook wouldn't annoy me in a future endgame, but I was missing that losing a very active piece was making that an endgame was probably not going ever to happen. Thanks for the explanation. :)
PD: As a curiosity, in that game Black's next move was not Qxa8, but Nxd4, so I could save my bishop. :P
In my last game I had the following situation (I was playing White):
I played Bxa8, and according to the computer that was a blunder; I had to move Bxc6. Okay, I realize that if I moved Bxc6 I could have gained a knight without sacrificing my bishop, but then Black could have taken away his rook, and I thought that it was probably worth more to exchange a bishop for a rook than just gaining a knight. What am I missing here?