Because bigger jets were too rich on my bike. Seems obvious!
If you find the official Mikuni VM set up procedure (it's available online, Google it) it tells you how to assess whether the pilot jet is the right size. As I said, I was opening the air screw more than optimal (4 turns out or more) to get correct running, so you go down a size.
As I also said, I get zero popping.
Bear in mind your bike, exhaust, air filter, fuel, atmospheric conditions are all different to mine, so set ups are not necessarily transferable. Hence why you need to be able to fine tune the carb yourself.