In a team of 10, you have 10 x (10 - 1) / 2 = 45 communication channels

In a team of 15, you have 15 x (15 - 1) / 2 = 105 communication channels

Which implies that the time you need to spend communicating would be around double. So if you need 1 hour meeting per day in a 10 person team, you will need 2 hours of meetings in a 15 person team.

The only solution that I know, however impopular, is to form a hierarchy of information, just like you would organize a growing code base in a hierarchy of abstractions.