Yep, and even now I can't go off the costume size for my rugrats. My older two are 11 and 8, but their upper halves don't always match their lower halves for any kind of clothing. If the costume is two pieces, and it says Medium, great! My oldest can wear the top, and my younger can wear the bottoms... maybe. Weight and measurement is always a good call.
And if in doubt, order in advance so you can return/exchange if need be. Or, find a local halloween store, test the size out, then pay less online.
Don't look at me! I'm just the passenger!