I have recently been listening to Beethoven's Oboe Trios. It sound like a orches!

I have recently been listening to Beethoven's Oboe Trios. It sound like a orchestra but is only three. How?

The question is interesting. Going after the previous answer, if a few instruments are playing different notes, the wider the span between highest and lowest, the more you'll tend to "fill" in the missing harmonic notes. It's a psycho-perceptive effect. The contrary is also true; take a very compact line (e.g. the beginning of Tannhaeuser ouverture) played by a large section of orchestra, and it'll look like a chamber ensemble.