The diapered/diaperless thing makes sense. I know that if you go to various clothing websites or look at clothes patterns, 2T is *fractionally* larger than 24 months, but much less than the difference between 18 and 24 months, or 2T and 3T.
Starting at 2T, my girls have always worn that size for a full year. Just recently at sizes 6 and 4T, I found that two of them actually wore the clothes for closer to 2 years. So it is nice not to have to worry so much about seasons.
I'd never noticed any problem with heads fitting through neckholes (in fact, I often find that clothes have a bit of stretch and you can fit the child's head through the neckhole without unsnapping a snap, if there is one). However, I know that I apparently have a larger-than-average sized head, because your typical "one size fits all" hat is too small for me, and the same thing goes with anything that is fitted for the head (goggles, etc.). My dad is the same way.
My mother tried to order a hat for my dad, as he too couldn't fit average one size fits all hats, and it still turned out too small. So she called the company and the woman said her husband was the same way and by the way, was mom's husband Irish?" Why yes...apparently the woman on the phone also married an Irish guy with a big head. Hm!
Anyway, back on subject, it seems sweaters are often the ones with tight neck holes, t shirts tend to be pretty elastic.
How interesting! I wonder if that's really true. Because my Dad has full Irish heritage (both parents and all grandparents, etc.), whereas I have half Irish from him. I should ask him if he knows if anybody else in his family has the big head problem.
It does seem like my girls didn't inherit it, though. Although, they're all small for their age, which I was a little kid, too (I just kept growing longer, which is why I'm taller than average now), so the head thing might not come up yet.
I agree with Emmy, that generally I think that "6 months" is the same as "0-6 months" and "24 months" is the same as "18-24 months," whereas 2T is the same as "age 2," and that's exactly how my kids have fit into their clothes, or at least until recently. When my oldest was 3, she wore 3T, etc. With some clothing items, going up a size was fine, particularly shirts or dresses. Pants, which are fitted at the waist, are less forgiving.
I do have a friend whose kids wear mixed sizes, something like 5T shirts and 2T pants, or something crazy like that. I don't remember which way it goes, I think the pant size is smaller, but I'm not exaggerating when I say they're something like 3 sizes apart. Of course, this particular friend is also about 5'1" and 110 pounds, and her husband is only a few inches taller, so her kids are almost falling off the bottom of the charts in terms of size, so that probably affects their sizing (when you have a really short 5-year-old, he isn't going to be quite the same body proportions as a 3-year-old, despite being the same height).
I've also heard the same as StarDrifter about different brands sizing differently. Especially in the baby clothes, I think it's Carter's that is known for being long and skinny. But you'll have some that tend to be long and skinny for the size, others that are short and stocky. Or a strange mix therein.
I do remember being a kid and wearing about size 6 shoes (probably kid sizes, not adult). But there was one pair of shoes I had in size 10! And they were nearly too small. It was just the fit of those particular shoes. I remember they were Cherokee, because I was delighted at the name because I was slightly obsessed with the Native American tribe at the time.