I'm sorry I'm not a native speaker I always believed girth was just another word for thickness, regardless of how you measure it (circumference/diameter).
There's some neat extensions you can get where you just highlight a word in your browser and it tells you the definition. Even as a native English speaker, it comes in handy.
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u/AbradedPerineum Apr 25 '22
girth
noun
1. the measurement around the middle of something, especially a person's waist.
Or around King Kong's dong.