A project needs help, so more people are added.

That sounds reasonable.

But as the group grows, meetings get longer, communication becomes harder, roles begin overlapping, and everyone becomes slightly less certain about who is responsible for what.

That is the Ringelmann Effect:

As groups grow, individual effort and coordination can decline.

Adding five people does not automatically produce five people’s worth of additional progress.

Larger teams require clearer roles, stronger coordination, and visible accountability. Without those things, people can disappear into the group.

Sometimes the team does need more help.

Sometimes it just needs everyone already in the room to know why they are there.