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.