The ever-popular virtual idol Hatsune Miku is probably the Nendoroid with the most incarnations in both regular and petite sizes. Because of this, I have selected Miku as a standard for comparison to other petite figures.
Most of the Miku Nendoroid Petits that I own seem to be roughly the same size. The figures in the following image include Saihate (サイハテ) Miku, and the 2009 Racing Miku.
Cheerful Miku is similar in size, though her body proportions are slightly different due to her pose. If she were in a standing position she would probably be at least 70mm tall because of the length of her legs.
Hachune Miku (はちゅねミク) is also the same size as the other Miku figures, but again her proportions are a bit different. Her head is smaller, her body more compact, and her legs are longer.
Hachune Miku is actually one of the nicest Nendoroid Petit figures available because she has so many points of articulation (knees, hips, shoulders, elbows, neck, and hair).
One of the exclusive Miku figures that I own is the Project DIVA Miku from the PSP game. The sculpt is slightly larger than other Miku petites, with head:body proportions that match those of a full sized Nendoroid more closely.
Another exclusive figure is the 2010 Snow Miku commemorating her hometown Sapporo Snow Festival.
As can be seen, Snow Miku is a re-paint of the Project DIVA Miku. They really need to do a Sakura Miku re-paint of this figure as well.
I really like this years Snow Miku design, but unfortunately, no petite version of the Nendoroid was made.
In closing, I present one of the videos I saw at the Vocaloid: Sound of the Future Panel during AX2009 that finally got me to take a closer look at Vocaloids, and made me a fan of Miku.