Why does Sonos connect need to be that big?

Probably heat, at the time it was designed, what, 10 years ago? Plus I suspect it needed to be the right size to fit next to something on a shelf, since probably the greater majority of people weren’t mounting it behind a TV set?

Not sure why you’re mounting a CONNECT near a TV set in the first place, it’s not designed to work with a TV set. It can, though, just not what it was designed for. 

Why cant I use it with a TV?

If the screen is very very big (85 Inches) and I dont want a lot of sound near the tv.. just at the other end of the room next to the sofa, why would I go for a Playbar and fill the entire room with sound. The volume will have to be really loud to be able to understand the dialogues.

Was thinking of ceiling-mounted or wall-mounted speakers.. or maybe 2 play one just near to the place where the seat is to keep the overall volume low.

The 75ms delay on the analog line in. It would make any lip sync well nigh impossible. But try it before taking my word, and mounting it. You may find it acceptable.  But it wasn’t in the designers / product person’s mind when the device was created, to be sure. 

Thats a good point.
I think I will rather skip sonos and hardwire the ceiling speaker with a regular amp.

Presumably one that fits behind the TV?

You might go with the Sonos Amp, instead, which has a Digital HDMI ARC input, without the line in delay. I am pretty sure it is smaller in stature, too, so it might fit behind your TV better. It was deigned for use in home theaters, whereas the CONNECT was not.

with a Connect you would still need a regular amp anyway. The neatest solution would be a Sonos Amp.