I copied this from what to expect website. It was my email today.....
Are a good fit. Here?s how toddler clothing sizes break down according to a child?s height and weight:
2T: 25?27 lbs., 32?34 in.
3T: 28?32 lbs., 35?37 in.
4T: 33?36 lbs., 38?40 in.
5T: 37?42 lbs., 41?43 in.
That?s generally speaking, though. Some other things to keep in mind regarding
fit:
If
your child is on the short side, she may be able to wear clothing labeled in
month sizes (for a baby) rather than in the T (toddler) size. Month sizes have
the same weight range but a slightly lower height range.
European
sizing is based on how tall tots are, so convert your child?s height from
inches to centimeters (you can try this calculator) and you?ll always know exactly which
size to buy.
If
the toddler size of an outfit you like is out of stock, the kid size may well
fit your small fry: Kid sizes tend to be longer with less room in the seat (toddler
sizes allow space for a diaper), so a tall toddler who?s totally potty trained
can probably pull off a kid size.
Swimsuits
typically run small, so you?ll almost always need to buy one size up from what
your tot usually wears. (If you buy a bigger size in a one piece for a little
girl, you may need to fasten the straps together in the back to keep them from
slipping down her shoulders.)
If
you want to spring for a fancy outfit from a high-end kiddie boutique (for a
special occasion, say), go for it ? but be aware that these tend to run small,
too.

Re: sick of having to go exchange tot sizes...
Most of R's clothes are still in the 12 month range, and are still pretty loose. But we also have some 18 month clothing that fits her pretty well. I've found it also varies on brand. Thanks for the good info Red!
ETA: R is 22 lbs and 30 inches. Right in the middle of her age group!
Ok, you guys are making me feel like my kid is a freak of nature. I put DS in a 12mo onesie today and it fit him perfectly.
I'm having a hard enough time processing this, could you knock it off?