DURING THE EVOLUTIONARY HISTORY OF ANIMALS THERE HAS BEEN A TENDENCY TOWARDS INCREASE IN BODY SIZE EXPLAIN WHY?

There have not been always a tendency towards the increase in body size during the evolutionary history of animals.  

The tendency of body size to increase over time remain poorly understood.

For e.g. Horse evolved from a species of height of just 28 cm to present day of about 150 cm whereas Dinosaurs (extinct species) were vary large as compared with their existing representatives. 

Indeed large body size is considered a population characteristic of species more susceptible to extinction.