The big ones are also diesel, run at different RPM. ~3500 RPM For the smalle gas ones and around 1800 RPM for the big diesel ones. The noise characteristcs are also different. The big diesels produce more low frequency noise, whereas the small gas generators are literally lawn mower / leaf blower engines and sound the same.

The enclosures of the big generators remove mainly lower frequency sound, making them perceptually even quieter.