While this is smaller than other sleep sacks, this is intentional and there is no need to size up.  If you size up, the neck and arm holes will be too big and your baby will be lost in the sack.

This is my favorite sleep sack partially because it is designed to fit babies more snugly ... not so much extra (and unnecessary) fabric for the baby to tangle in.

The main reason this sack is my favorite, though, is because it has a two-way zipper that makes dressing and diapering my daughter a DREAM compared with the regular top-down only zipper sacks (all the other brands, from what I can tell ... I've got 2 other brands).  With a top-down only zipper, getting my fussy baby into this thing before bed is a challenge, to say the least.  With this, though, No Problem!

Of course, the fact that this is so soft and snuggly doesn't hurt any.  I wouldn't mind having one of these in my size!  :)

The only drawback I found to this sack is that the neck hole isn't V-ed at all to keep the bulky fabric away from the baby's face area.  I was worried that this might get in the way of our daughter's mouth while she was little tiny, so I didn't start using this until she was 3+ months old.  Now that she's a bit larger and has more control of her neck I feel okay leaving her in it.  Anyway, this is partially why you would definitely not want to size up on this sack ... you want the sack to be pretty well fitted around the neck and shoulders.

The lack of a V neck-line is the only reason I give this 4 stars instead of 5.  I love it!!