The answer is like the one proffered by Hazoriz, only it's a little muddled because he misquoted some details.
The halacha about what is considered whole regarding a Matza is no different than the question about how much can a bread be cut through and still be considered whole for the purpose of "Lechem Mishna" - the mitzvah of making a blessing over whole bread and eating from it on Shabbos.
The Kitzur Shulchan Aruch states that even if a loaf is cut part of the way through, it is still considered whole for the purpose of the Mitzvah, as long as you can hold one end of the loaf and the other end will not break off.
So too with Matza, it doesn't matter if it has a crack - even a large one, as long as the rest of the Matza would not break off if you were to hold it on either side of this crack, the Matza is still considered whole for the purpose of the Mitzva on Pesach.