In making an amigurumi whale recently, I made the unfortunate discovery that Lion Brand Jiffy yarn can vary in size from one skein to the next. I'd suggest keeping this in mind when selecting a crochet hook to use for this brand of yarn. (I or J seem to work best for me.)
I had to used a smaller hook size for the new whale I made because the newer Lion Brand Jiffy yarn skein I was using wasn't as bulky as the yarn from the skein I based the original pattern off of. It wasn't as soft and fluffy, either... :(
For comparison, I've taken a photo of both whales. The larger whale is the original whale I made when I first designed the pattern. The smaller one is the more recent creation.
I really am hoping that this is just a random fluke and that Lion Brand Jiffy hasn't changed the entire composition of their yarn. But, since I think all but one or two of my patterns use Jiffy yarn at this point, I felt it would be important to point this detail out.
Happy crocheting! :]