Think about it this way....
Hot exhaust gasses have more volume then cold exhaust gasses. As the gasses cool going through the pipes they contract, they actually require LESS pipe volume to flow the same amount of gas. This is simple physics. That's why the manufactureres can use smaller tailpipes. They've done their home work.
Splitting a single 2 1/2" pipe into two 2 1/4" pipes almost doubles the pipe volume for spent exhaust gasses that don't need as much volume... You could easily use two 2" pipes and not see a restriction... You don't need 2 1/2"s.