What does Boxing Day have to do with boxing?
development at an increasingly rapid rate in proportion to the growing total number or size; a constant rate of growth applied to a continuously growing base over a period of time
Growth of a system in which the amount being added to the system is proportional to the amount already present: the bigger the system is, the greater the increase. (See geometric progression.)
Note: In everyday speech, exponential growth means runaway expansion, such as in population growth.