That's my question. Have you been catching bigger fish on the 6" than you have on the 4"?
In my experience, they will both catch big fish. The thing is that some day's 1 will catch them when the other wont. With Gulp, it's important to change color and size to see what will work on any given day. The small shrimp or crab may be what they want, if you use the same thing every day, you'll have days when you don't get a bite.