Other things to consider on sizing:
Depending on the item of clothing, the critical dimension can vary. Shirts tend to be relatively forgiving, but boy clothes tend to be less fitted, and girls tend to be more fitted. Raglan shoulders are also more forgiving than defined shoulder locations. Otoh, my kids often outgrew onesies in torso length first. Overalls and rompers don't have to fit at the waist, while pants are the most difficult because both the waist and the length need to fit at the same time. (Of course, you can always hem, or take in the waist.) Dresses are the most forgiving (unless they are very fitted).
And, once you get past 18 months, forget about sizing consistency, and welcome to size ranges. Not to mention that some clothes you will find in the infant section, and some in the toddler section, etc, depending on the store. Small, medium, large, x-large mean different things depending on whether the clothes are meant for toddlers, little kids, big kids. It can get maddening! And of course, when you buy lots of things used, you don't get the sizing chart help, either - you just have to hold things up and see if they seem true to size. Tho at least you probably won't get shrinkage that way!