Okay, so I e-mailed Steve Hartman to try to get more details on the "slightly larger petite" but I didn't hear back from him for 10 days so I called them today. I didn't speak to him but the woman I spoke to was very knowledgeable and was able to answer my questions.
She said they re-vamped the entire line of petite and x-small sizes and made them slightly larger. I made sure she meant the collar was larger and she said it was. I asked her how much larger the collar was on the petite and she said it was 1/8" larger.
She told me if I ordered through the website to specify I wanted the "new, larger petite" since they still have some of the old ones. She didn't know if there was a comment box so she said after I placed the order I should e-mail them with the order number and let them know via e-mail that I wanted the larger one.
I mentioned this earlier in the thread but they do free exchanges so if you have a petite that's too small for your GCC you might want to send it back and exchange it for the new one since the collar is larger.