There is no serious science in these kind of mufflers. They are there to reduce noise levels, and manage heat/fire issues of exhaust gasses.
The truth is it doesn't matter much how many holes and in which places you make, you aren't going to achieve anything (past "letting it breathe, exchanging some minor gains in return for more noise) until you properly pipe it. One hole, two holes, 10 holes, weld a 2" pipe to it, any kind of deflector, it won't make any real difference.
Size of the muffler is probably designed by how much space they have left. The bigger the better, as it helps cool down the gases some, would be my guess.
The truth is it doesn't matter much how many holes and in which places you make, you aren't going to achieve anything (past "letting it breathe, exchanging some minor gains in return for more noise) until you properly pipe it. One hole, two holes, 10 holes, weld a 2" pipe to it, any kind of deflector, it won't make any real difference.
Size of the muffler is probably designed by how much space they have left. The bigger the better, as it helps cool down the gases some, would be my guess.