My 6 year old was able to fit into a 3 year old's clothing?

My 6 year old niece was able to fit into her 3 year old cousin's pajamas. Her suitcase didn't arrive when she did and we had to find any pajamas we could, so she tried on my 3 yr old son's pj's and they were a perfect fit! They are age 3-4 (too big for my son, so were abandoned in the closet). Do you think kids are getting smaller these days? :S