Shifting scale: the team chat room

Situation: a team chat room used by 10 people. About 200 messages a day. Announcements, work discussion, and small talk are all mixed in a single channel, but for now everyone reads everything, and any missed context can be sorted out by asking the person next to you.

Imagine this chat room's headcount becoming (1) 10x, at 100, (2) 100x, at 1,000, and (3) 1/10, at 1 (a channel just for me) — each in turn. For each scale, describe separately 'what breaks' and 'what newly emerges.' 'More messages / it goes quiet' is not an answer. Pinpoint how reading, writing, trust, and rules change qualitatively at each scale. You must address all three directions.