I wonder why everyone's metric for success is always Eurocentric. Some might think Blackpink are bigger because they have an audience among "Western", American or European countries but that doesn't necessarily constitute a global audience.
Kpop spread across Asia before it could reach the West and the Hallyu wave has been a slow-moving soft power process since the 90s (arguably). Arguing that Big Bang is the biggest because they're probably more well-known across Asia is not the far reach people think it is, especially when you add the element of their longevity and the fact they were able to access a wider audience (across genders and ages etc).
When people select highest-grossing movies of all time, people tend to adjust for inflation. It makes perfect sense why some would apply a similar way of thinking when discussing who's the "biggest" as all groups listed are from different generations so to make a direct comparison becomes rather difficult. If this question had the option for PSY, he would also be a strong contender for being the biggest in this line of argument.
Really the question should have been framed better- does biggest really mean a group with a Western audience? Are we referring to groups biggest within Korea? Or who has the biggest audience across all age groups? Or does it mean biggest in terms of revenue and income generated? Does it mean who is the biggest in 2024 or who is the biggest with adjustments to the limitations of their respective (Kpop) generation?