Compression is a bit of it. The other bit is commercials usually mix in stereo and negate the 5.1. By not using the subwoofer, it gives an extra 10dB of immediately available headroom, before any compression or limiting occurs. 10dB is... you guessed it, more than double the amount of perceived loudness.

The solid rectangle block is the result of using a limiter. Basically a hard ceiling compressor, for simplicity.

source: been there done that