Winchester evidently considers their 375 Win lever action as “Big Bore”. They named it such. There’s a book on “Big Bore” cartridges, it covers 8mm and up. Al Miller wrote 358 with 250 grain bullets and up...
So as your post implies the title “Big Bore” is a definition that is somewhat different depending on one thing, “Opinion”.
To me consider a 416 Weatherby, well if that isn’t a big bore in cartridges available today, with the tremendous energy it produces, well???
Compare that 416 to a 50-110 or a 45-65 or a 54 muzzle-loader.
I’m not sure any of my rifles would be considered a big bore. But my 54 muzzle loader looks like it has a mighty big hole at the end of the barrel. My 416 loaded to almost 6,000 ft-lbs of energy feels like a big bore to my friends who have fired it.
Well we all have opinions and this post shows that clearly.