Some of the modifications that I made included casting on 38 stitches for the ribbing and increasing to 48 for the hand portion; starting the decreases for the top of the mitten on row 38; increasing the thumb gusset to only 17 stitches; and starting the decreases for the thumb about 6 or 8 rows before what the pattern calls for.
Full disclosure: I ripped out the top of the mittens, after they were completely finished, because they were huge - much too long, even for someone with more normal-sized adults hands. And I did the same thing with one of the thumbs (but not the other, I couldn't find the end of the yarn and I was just ready to stop knitting these mittens.) As often happens, blocking places enough time and space between knitter and product for you to be able to fall in love with the end result and forget all the trials. Once they were completely dry, I thought, now that's a good mitten!
I'm happy with these and may keep them, as they are still a bit wee for most adults. I'm starting a new pair (this is my new knit-in-the-car project) with 44 stitches cast on for the ribbing, increasing to 48 for the mitten, and working all but the ribbing on a size 5 needle. I'm hoping that it makes the mitten just a bit larger and decreases the effort of working it on a smaller needle.