It all depends on the mission! Sure, capsules can be as small and light as needed! Soyuzes weigh less than 8 tons.
There will always be as many seats available to return back as there were going up. This wasn't the case with the Shuttle as it was once suggested they could leave a crew aboard a station and come back.
That's when a rescue capsule (like the X38) would have been needed. With the shuttle gone, no need for a rescue capsule! Especially when the Soyuzes are designed to stay as long as the astronauts.

Is it possible to build a capsule for 6 astronauts within a spacecraft weighing not more than the Soyuz? That's what interests me!
Can the private sector without NASA supervision perform better than a contracting with NASA private sector ? Maybe as well, but I'm sure not better!