I'm wondering why, when shorty headers that have 3" collectors have corresponding y-pipes that are only 2.5" into 3 at the cat end or in some case 2.25" into 2.5"? For high HP cars it would seem that 3" collectors would like a 3" Y-pipe. Otherwise why not make the collector 2.5"? I'm sure there are good reasons for this.

 

Packaging.

It's a tight fit to make a 3" y pipe. Esp where it goes between the k member and the oil pan.
Mass production 3" y pipes would likely end up with y pipes that gave clearance issues.