Bigger Bombs is the first upgrade to the Bomb Tower in Bloons Tower Defense 4, the first upgrade of path 2 in Bloons Tower Defense 5, and the first upgrade of Path 1 in BTD6. As it says in the description, this upgrade allows the tower to fire bigger bombs that affect a larger area, popping more bloons (50 in BTD5 and earlier, 28 in BTD6).
Bigger Bombs looks very similar to the regular bomb tower. All that changes is that the bombs turn red, and the appearance of a target on the back of the Bomb Tower. After all, all that really changes is how big the explosion is.