This seems unlikely, but it’s the only thing I can think of that might account for that much excess: any chance something went wrong with the placket? The set-in placket option has you removing a strip from one of the shirt front pieces – if memory serves it’s a 1″ strip.
I’ve made the classic shirt in a 10 (with alterations, but not to the neckline), and I don’t remember any difficulty fitting the collar stand to the neckline.
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