Alright, I got an answer from them.
Hello Jessi,
Thank you for your inquiry. While Cockatiels do originate from Australia, we find that Cockatiels bred in the United States are slightly larger & tend to fit our size MEDIUM better. Customers in the States who refer to their Cockatiels as being “a little smaller” will fit our size SMALL. On the other hand, our Australian customer’s Cockatiels will almost always fit our size SMALL.
If you are unsure of your Cockatiel’s size, please visit our website to see a pictorial tutorial on how to measure your bird’s girth and length for sizing -
http://www.flightquarters.com/bird-diapers/FAQS.html Just click on the question “How do I measure my bird?”.
We hope this helps. Please contact us again if you have further questions or need assistance with ordering. Have a great day!