craftgrrl | Modifying Crochet Pattern Sizes

Mostly, this is in reference to crochet toys/amigurumi.

I usually follow patterns, or work with my own patterns, and make small to medium sized items. But, I've had a friend request that I make one of my amigurumi pieces for them -- but four times its size!

I don't have experience with enlarging patterns, but I know that if I work with a thinner weight yarn and smaller hook, the proportions stay about the same and I have a smaller finished piece.

Theoretically then, I could use thicker yarn (or in this case, since he wants it so big, multiple strands of yarn held together) and a larger hook -- to make it larger? Would this still come out proportionally the same? Or would it be better just to try and completely modify/re-write the pattern to create a bigger piece?