Really Big Bombs is the third upgrade of Path 1 for the Bomb Shooter in BTD6. It allows explosions to become slightly larger, as well as popping 48 bloons per bomb instead of 28. When crosspathed with Frag Bombs, fragments have +1 pierce (2 pierce), last longer, and have larger hitboxes.
It cost $1,020 on Easy, $1,200 on Medium, $1,295 on Hard and $1,440 on Impoppable.
Really Big Bombs makes a good stepping stone to the more powerful Bloon Impact, but alone it is not as good as a 2-0-3 Cluster Bombs, as it doesn't have the bonus range or bonus explosions that can inflict more DPS.
One advantage of Really Big Bombs compared to a 2-0-3 Cluster Bombs is that it can have both the benefits of Path 1 and Path 2, for the larger explosions and the faster attack speed. However, 2-0-3 Cluster Bombs usually performs better overall for a lower price.