There is something wrong with the detail size (sculpt mode) in my file. At first it worked properly, but i don't know why it started doing this:
It seems like, the detail size value is inverted, but i don't know why. With 20px resolution it makes much more polygons than with 3px, and it should be just the opposite. I don't know if i pressed any buttons by error or if it's a program bug.
I found a solution, that is copying all the stuff and paste them in a new file, and now it works ok again. But i really would like to know the reason for this problem, and if someone could tell me another solution, 'cause now i have to set again the background images in the correct place.
Thanks a lot