Where the word comes from, what separates amigurumi from ordinary crochet, and why the dolls look the way they do.
The word
Amigurumi is Japanese: ami, crocheted or knitted, and nuigurumi, stuffed toy. Put together it means a small crocheted stuffed figure. The craft spread worldwide in the 2000s and the word came with it.
What makes it different from other crochet
A scarf is worked flat, in rows, and you want it soft and open. An amigurumi doll is the opposite on both counts.
It is worked in a spiral, not in rows. There is no seam up the back, which is why the dolls look continuous.
It is worked tightly. Deliberately tighter than the yarn suggests, because loose stitches let stuffing show through. This is also why the fabric holds a shape.
The shape comes from counting. Increases and decreases in the right places make a head round and a limb tapered. There is no pattern piece and no cutting — the form is arithmetic.
Why the proportions look like that
Large head, small body, short limbs. Partly tradition, partly practical: a large head gives room for a face, and a face is what makes a doll a character rather than a shape. The Beddy Byes crochet doll set is a clear example — nine characters, each recognisable almost entirely from the head.
Handmade means slightly different every time
Tension varies between one sitting and the next, so no two dolls are identical even from the same pattern. That is not a defect; it is the difference between a handmade object and a moulded one. More on the time involved in how long a handmade crochet doll takes to make.
See the range in cartoon crochet dolls.