also, the more people on a team the more people who need to agree on something, meaning that more time is spent on discussing. This also means it takes longer to decide which features to cut once it turns out that the game idea was over scoped (which always happens), meaning more time wasted during a time that there is NO time to waste.
It can also create bottlenecks, if someone ran against a wall while implementing a core feature, or similar. Adding to wasted time.