I think, much like FaceTime, it's designed primarily (both UX and architecture) around the two-party call use case, but can support several more as needed (12 total for Duo, 32 for Facetime).
Whereas solutions like Meet and Zoom are designed primarily (both UX and architecture) around multi-party, supporting 100's (250 for Meet, 500 for Zoom).
Note that intuitively, it seems like Meet and Zoom have the better model - why limit the number of participants when you don't have to? And in fact a lot of consumers seem to feel this way about it; many people I know use Zoom for everything, from large meetings to chatting with a small number of family members or confidants.
But under the hood, limiting the number of users allows some important technical advantages. Duo claims (in its FAQ) to be end-to-end encrypted. I believe Whatapp claims the same. And it wouldn't surprise me, knowing Apple, if their video chats on Facetime did too. This is possible only with a reasonable limit on the number of participants because of the need to negotiate a bandwidth for each stream that works for all viewers, and each stream needs to be independent, not multiplexed into a single live view.
The proper use of large-scale meeting products is in corporate or paid environments where you have some kind of guarantee that your chats aren't being recorded or sold. (At least unless a three letter agency is involved.) Tools that guarantee end-to-end encryption should be used in small group chats.
Your point about UX is a strong one too. There are a affordances that work better for smaller chats, others for larger ones, and it's helpful to be able to specialize.
> The proper use of large-scale meeting products is in corporate or paid environments where you have some kind of guarantee that your chats aren't being recorded or sold.
In banking/financial services, we want guarantees that all chats are being recorded. In lots of other regulated and government fields too.
> why limit the number of participants when you don't have to
Because beyond UX, there are a ton of technical tradeoffs involved.
In a 2-person call you want to avoid a server if possible, because it adds latency -- so that's a good first reason. Zoom and Meet would never dream of not using a server.
The optimal architecture of a "small group call" is totally different from a "large group call".
Whereas solutions like Meet and Zoom are designed primarily (both UX and architecture) around multi-party, supporting 100's (250 for Meet, 500 for Zoom).