Modern Vespa

My ET2 came with a Weber 12mm with I believe a size 75 main jet. I replaced the exhaust and added a big bore and found an 82 was appropriate. Now, I've moved on to a Dellorto 17.5mm which seems to be happy with an 80.

So does a 50% larger carb mean 50% more fuel with the same jet? Is there a rule of thumb for jet size increases? For example, does a 12% increase in jet size (75 to 84) equal 12% more fuel flow? Talking peak flow at full throttle.

I'm assuming it's not that simple. Trying to wrap my head around carb/jet sizes and how they compare.

It's a whole bunch of variables, some of them more obvious than others.
1) The area of the carb's intake
2) The area of the jet's opening
3) The peak power RPM's
4) The cylinder displacement

intake area and jet area describe the fundamental ratio, but RPM's and displacement determine the vacuum draft, which in turn defines how much fuel is actually flowing.

So answering your second question first, the AREA (not diameter) of the jet's opening to the area of the carb (when holding peak power RPM's and displacement constant), is fairly linear.

(I literally built a spreadsheet to model estimated changes and tested them with an AFR over the weekend)

I didn't try to model the impact of a change in carb size, but playing the https://kyajet.de and testing it out, they predict that changing from a 28 to a 38mm diameter carb (basically doubling intake area) only reduces the mixture ratio by about 5% at the main, but 10% leaner at half throttle and 15% leaner at 1/4 throttle, so go figure.

I'm not going to do the geometry to work out actual area for a segment of a circle like that, but I suspect it will be basically linear there, too.

Each carb has an optimal main jet size. If a factory fitted carb has been selected correctly by the manufacturer, the main jet size will be within a small percentage of it's optimal size.
When a carb is too small for an engine, the main jet has to be disproportionately large to compensate. If your project needs a main jet way bigger than a factory bike with the same carb, it's a safe bet that the carb is too small for your project.
The OP provided a prime example. Much bigger carb, yet required a smaller main jet. And that smaller main jet chosen, is within the optimal range of that carb.