A recent update for Overwatch brought together new game modes and an update which has been loved by the community so far. The Retribution event update, however, is a massive update with the quantity running upwards of 15 GB.
We do, and that's why this is large. Occasionally we need to repackage everything so future patches can be smaller. ^KAL
— BlizzardCS (@BlizzardCS) April 10, 2018
While the community was surprised with this, a lot of the download size was intended, according to Blizzard developers. They wanted to reorganize the game files so that future game updates can be smaller and organized better.