An engine can only pump so much air in and out. Larger carb does allow more air at a given moment, but it also slows down the velocity of the air going in. There's a point with every engine where a specific velocity is most efficient at providing the right amount of air at the right time (read: produces the most power). The 42 provides plenty. It's just possible to improve the delivery of that mass of air with a smaller carb and not be too small, and actually get more power than the 42 across a larger range of rpms because of the increased air velocity. Jetting properly allows that air mass to grab whatever fuel it needs at whatever velocity it travels through the carb.

And y'all been wondering why PVT recommended a 38mm for years

That 38 doesn't flow what these lectrons, the flat slide mikuni, nor the keihin d slide will flow for equivalent size. It runs out on top before those others will, even with the flow plate thing for the slide he suggests to decrease turbulence.

Just a simple equation will get you roughly in the ballpark for how much cfm you need.

Variables as far as air temp, combustion efficiency, flow characteristics of intake, valves, etc will have an effect.

RPM X CID / 3456 and at what percent efficiency.

So for example take 4500 rpm x 61 cid /3456 x .8 = 99.283854 cfm