Neither one of those is correct. They like to over estimate the "acceptance volume", as a sales feature. It depends on the air charge and pressure switch setting as to how much water a tank will accept. With the correct air charge and a 40/60 switch, an 86 gallon tank holds 23.03 gallons while a 119 gallon size tank holds 31.86.
Now if you go up to a 50/70 pressure switch setting the 86 gallon tank holds 20.31, and the 119 gallon tank holds 28.10 gallons.
If you go down to a 20/40 pressure switch setting an 86 holds 31.44, while the 119 holds 43.51 gallons.
Either way your water doesn't come from the tank, it comes from the pump. The only reason to increase the size of a pressure tank is to reduce the number of on/off cycles, so the pump doesn't get destroyed prematurely. There are better ways to stop the cycling, then you don't need a large and expensive tank.
Here is a picture of a pump system with an 80 gallon pressure tank (white tank in corner) that feeds a city with over 100,000 people and has three 200HP pumps.
View attachment 44075