It's recovering from being unhappy, I don't know that I would call it growth, but yes, leather corals can change size quickly.
The first photo shows a very unhappy leather coral, next photo shows a slightly more happy leather. It should get even bigger as it gets used to your tank and "fluffs up". (<-My scientific term right there!)
It kinda looks like it was on a piece of live rock that was turned over, shading the coral for a while, causing it to decline, and now it is recovering.