You bring home a dog, it grows up with you, you get attached to it, and sadly lose him in a few years (which is too soon) - this is what happens with bigger dogs. The lifespan of a big dog is around 8-10 years, and that of a small dog is 12-14 years. Small dogs live with you longer than big ones, in most cases.
So these were the many reasons why small dogs are a better bet. But when we talk about security, the size of a dog matters. The size of a small breed dog can be one of the cons in such a case, but there are so many pros to cover this. Mind you, big dogs are very cute and affectionate too, so let's not discount them completely.