Guide
Symphony Orchestra vs. Chamber Orchestra
The same instrument families, at very different scale.
The distinction between a symphony orchestra and a chamber orchestra is mostly a matter of size, but that size difference shapes almost everything else about how each one sounds and what it's suited to play. The name echoes chamber music for the same reason: a smaller group, suited to a smaller room.
Size
A full symphony orchestra typically runs somewhere from 70 to over 100 musicians, with a large string section (often 30+ violins alone), a full complement of woodwinds and brass, and an expanded percussion section. A chamber orchestra is much smaller, generally somewhere around 20 to 50 players, with proportionally fewer strings and often a reduced or entirely absent brass and percussion section.
Repertoire
Chamber orchestras are the natural home for Baroque and early Classical-era music, written before orchestras had grown to their later Romantic-era size — a Vivaldi concerto or early Haydn symphony was composed with an ensemble roughly this size in mind. Full symphony orchestras are built to handle the much larger scale of Romantic and later repertoire, including works that specifically require the extra brass, percussion, and string weight that later composers wrote into their scores.
Sound and venue
A smaller ensemble naturally produces a more transparent, detailed sound, with individual lines easier to pick out, and often performs in smaller halls suited to that scale. A full symphony orchestra is built for volume and weight, capable of filling a large concert hall with the kind of sound a Mahler or Strauss score demands.
Neither is "better" — each is simply the right tool for the repertoire it was built to play.