The biggest problem with running a large carb on small cylinder (especially on an single speed, fixed ratio transmission) is that you're really never going to get AS accurate tuning as you could with a smaller, proportionate bore, because your intake pulse/draw is weakened. Generally speaking, in flow dynamics you lose fluid speed velocity the greater your volume is. Big bore carb means air travels through it slower and has a slower reaction to throttle input changes because of this. In addition, many modern carbs are HEAVILY reliant on a strong intake pulse to properly meter
This is really true of mikuni carbs, where EVERY VM20 bike I have ever built or worked on whether for friends or Second Stroke/NYC Mopeds build, ran decently with the factory jets the VM20 comes with.
Now, I can tune it in from there, but their pulse metering is so good, they typically just work, unlike a PHBG.
Again, this can be motivated through many ways to get decent results, but in the end I will typically err to smaller carb per build.