In this case, the theme takes up the first 17 seconds of the piece. Then the variations begin. They are all uniform in length. Listen!
You can find many other sets of variations, such as:
The Harmonious Blacksmith Variations by Handel
Goldberg Variations by Bach
Variations on a theme of Haydn by Brahms
No doubt the most unusual set of variations are the Istar Variations by Vincent d'Indy. Istar was a woman who ultimately did a strip tease, and so d'Indy's piece begins with the most complex theme at the beginning (an implication of being clothed) and ends with the theme by itself at the end (indicating unclothed).
Album title: Elizabethan and Jacobean Music Label=Vanguard Classics; Catalog number=8102