After you beat the Arkaeyan Vault of Secrets and watch (or skip through) the cutscene where Kaos smashes the map to the lost city and goes back "Home" to his castle to have Glumshank's memory checked, Cali and Flynn mentioned that they blew up Kaos' floating fortress. ...But when? I played through Spyro's Adventure several times, and they never actually showed them blowing up the fortress. I remember that Kaos' chair blew up when he was trying to leave, and then it cut to a very dizzy Kaos who somehow wound up inside his fortress saying "Fear my giant floating head" in a delirious fashion, and then it cuts to Kaos being trapped in a bubble at Sanctuary before being sent to Earth. ...But they never showed Kaos' fortress being blown up. Doesn't mean it didn't happen, but they never showed it.
Like Spyro and RPs? Try this RP: http://forum.darkspyro.net/spyro/viewposts.php?topic=121906
Offscreen, not to mention a clear time difference from the final boss to the ending. The Skylanders were probably busy dragging Kaos back while Cali went with the explosives(since she uses them a bit in Giants and Trap Team).