The theory is that if you concentrate your forces rather than spreading them thinly, it has a stronger effect on the enemy.
F W Lanchester (1868-1948) put forward mathematical theories relating to combat in his 1916 book "Aircraft in Warfare: the dawn of the third arm" which suggested that it was necessary to outnumber the enemy by (I think) at least 3 to 1. This concept seems to have been taken into RAF doctrine, which would have influenced Leigh-Mallory.
Those who are better at maths than I might like to look at
http://www-users.york.ac.uk/~nm15/BoBtalk.pdf