Talking to KnitWit on the phone after the incident of the amazing shrinking teal hat, she asked me to impart some words of wisdom on what I thought went wrong. Not since House (MD) was asked to diagnose someone at the North Pole via Skype has such a task been requested, but here goes.
1: Tension was pulled way too tight.
Now, as someone who once made a pair of Fair Isle skull socks for Mr LouBug that could have doubled as body armour for the cat, I know how tempting it is to tug at those floats. This creates a fabric that has no stretch to it at all (and occasionally it has almost negative stretch). Hats often rely on stretch to fit correctly and no stretch would definitely take an adult hat down to a child’s size.
2: Accidentally followed the wrong size directions.
I haven’t seen the pattern, but I know that unless I go through it and carefully highlight my size I often end up with the wrong number of stitches/increases as my brain only seems to see the first set of numbers.
3: Yarn substitution went kablooy
Talking to her, I think number 3 is the most likely. Like most new knitters, she thought that yarn (in this case DK) could be simply be swapped for one of the same weight. If only life was that simple! DK, or double knit, is basically an 8 ply (or double 4ply) yarn and like with clothes, the fibre makes all the difference to how it will fit. The best way to picture it is to think in terms of dress sizes. A size 12 dress made from Lycra will fit different to a size 12 dress made from cotton, this is down to things like the springiness of the fibres (think of curly hair versus straight).
If you are planning to substitute a yarn here is a quick check list to go through.
1: Is it a similar fibre content?
Wool, acrylic or cotton will look very different in the final product in terms of drape, finish and stitch definition.
2: Is the yardage similar?
The closer the match the more likely the yarns will be the same in terms of springiness and stretch.
3: Is the gauge similar?
Another good indicator that the two yarns are likely to react the same way. You can fudge it a bit but if it is more than 3 stitches different then that will really stack up over a garment.
4: Check Ravelry!
The yarn page has thousands of people that have successfully substituted your yarn (as has the patterns page). Let the hive mind guide you!
Beyond that, I recommend trying it on as often as possible (or measuring it if it is not practical) and letting your gut guide you.