In regards to this, there are still highly manipulative practices that occur, that's with groups in excess of 15-20k members.
Most naturally grown Telegram groups consist of 5-15k members, before a sale is finished or finalized. Anything ranging from 20-100k, that was made post-2018, is consisted of fake, accounts pulled from other groups and other highly manipulative practices. These types of crypto groups you should always avoid.
Concurrently, there are a lot of "influencer" groups consisting of a few thousand members, with a crazy amount of activity. These are also highly faked telegram groups, running bots with fake messages and or paid messages for a specific time, to make the group appear active in order to sell AMAs or other public events for crypto teams. If you're serious about crypto in any way, then you won't be in these groups listening to some guy talking about the latest coin he's shilling. It's great for quick research, but always know these guys are in it for themselves, they are ultimately looking to profit from your voice, time and activity; just like any tech company in the real world. "Data is king."
All in all, when in doubt, view groups like Binance's groups, Binance's shilled IEOs and see how many members they have. Chances are, it will denote a reasonable Telegram group community size for any given period in the broader crypto market.
Also, as OP said, never invest based on the groups. A lot of shills, a lot of FOMO, a lot of nonsense and a lot more shady motherfuckers; all there to solely manipulate your thoughts. Even worse are the groups who ban users for giving critical feedback (in a non-shit talk way), because it may cause other users to ask the same questions.
The weapons they have to influence new investors or new people in the crypto world are with their social media groups such as telegrams. I have seen and found many groups like that with more than 50 thousand members and there are even some double groups that are distinguished only by the numbers group1, group2, group3 etc. and all the members they have are so many that even one telegram group can no longer accept members.
Just as you mean, when we as group members join and make critical comments they will not be happy, they will immediately block us or kick us from the group before other members think critically too and comment on the same thing, this is what they avoid. Admin and bot that they have very much and are always active.
From the many experiences that I have gained, a group with many members does not guarantee the project is genuine. They just use it to do "BIG ANNOUNCEMENT", whether it's for FOMO, large-scale project promotions with lots of discounts and more. The most important thing is to continue to do research first and discuss together with members in this forum.