That is an awesome video... the riders were top level guys. On the gas going into the corners and on the pipe! On the pipe means that they keep the power in the "meat" of the powerband. It takes some serious practice on a 125 to do that consistently.
If you watch closely many of them are on the gas going into the corner!
Another thing to notice is how the two-strokes almost "floated" through the whoop section and how the four-stroke seemed to pound through.
Andrew, I would guess that if your bike doesn't sound like that either you are not used to riding it in the meat of the power or the bike is not jetted correctly. Again it takes practice and big balls to ride fast through corners on a 125. Happy riding!