This power is used for one purpose: getting into the house. Santas a very hefty man (too many late night snacks), so its doubtful that a normal sized fat man is capable of squeezing through a chimney. However, with the ability to decrease the size of his body, Santa can shrink down to a smaller size and jump down the chimney with ease. I should mention that anything he touches also shrinks, so that sack hes holding gets just as small. Once hes in the house, he grows back to regular size and sets out the presents. Also, if any of the houses he visits dont have chimneys, he just shrinks and fits through the tiny crack between the houses front door and the ground. Problem solved. Wait, you ask, how does he get out of the house without anyone hearing him? Through a combination of two powers. He shrinks down again and uses his super-strength powered legs to jump through the chimney and back to the roof. If theres no chimney, he just shrinks down to go outside and then jumps to the roof. Either way, he grows back to normal size, gets back in the sleigh and continues his route.