From left to right: Kenner, Takara, Obitsu 21cm, Licca, Pure Neemo
There are noticeable differences in the shape of each doll but overall, the sizes are very similar. Liccas have very broad shoulders while the Pure Neemo bodies have wider hips and a thicker waist. The Obitsu 21cm body has smaller proportions overall creating a more childlike look.
I think each body has its merits.
The Obitsu bodies need minor modifications to support a Blythe head -- rubberbands or some sort of malleable plastic to support the neck joint and foam or felt in the hip and torso joints to support the overall weight. My Obitsu body still wobbles a little bit in the neck joint but I like the slight movement in the neck for expressions. With those modifications, the Obitsu body is excellent especially since it is so poseable and can often balance on its own. The 21cm Obitsu is slightly shorter than the other bodies.
The metal in the Licca bodies which allow the arms and legs to bend can become brittle and break with use. See SquirrelJunkie's arm repair photos for one example. My original BL Licca body broke a leg awhile back so as fun as they are to pose, they are not super durable. Liccas are a smidge shorter than Takara and Kenners.
The Pure Neemo are very sturdy feeling made with a solid resin plastic but the joints can easily slide apart. This is both a good and bad issue. Its good that body parts are easy to swap out since the original Neemo bodies are mostly rigid except for being able to rotate hands, arms and legs in a circular motion. There is some adjustment in the mid-arm and mid-leg to turn hands and feet in and out. I have the Flexion body as well which is slightly more jointed. The Neemo bodies are the tallest.
Kenner and Takara bodies are almost identical in terms of size and movement though the neck joins are different. Knees on both bodies click into place and arms only move in a circular motion. As they are considered the most authentically Blythe, they have their own charms.