Supposedly the XXS size on commercial patterns (McCalls, Simplicity,I had similar challenges sewing an infant shirt from a Butterick pattern. You are right that the sizing is just not right. You are also right that doll clothing patterns are not proportionally correct either.
Butterick) is supposed to be for babies 7 lbs or less, but they swallow
newborns up. If you read around the Internet, there are lots of frustrated
grandmothers and mothers-to-be that want to sew for their new or expected baby, but can't find a pattern that will fit. I read that doll clothing patterns don't work because the neck is wrong for a human infant. I tried buying vintage layette patterns from Etsy and they were just as bad. I want to know how to downsize a commercial XXS dress or romper pattern so it fits a NEW 7-8 lb baby.
There really is no easy way to fix the sizing issue. Perhaps the easiest would be to fold out some of the extra width and length to make the pattern smaller? Maybe try grading the pattern smaller -- though that won't solve the other problems that probably exist in your pattern. I can't really tell you how much to reduce the pattern because I don't know what pattern you are using. Even then, it would hard to advise you because I ended up redrafting the pattern I used above to even get it to work right.
One suggestion is to go to a thrift store and buy clothing in the size you want, take it apart and trace it off. Don't forget to add in proper seam allowances - as the existing seams have probably been trimmed off.
