On capacity and dimension numbers, cleaning area, machine width, walker length, etc, a 7720 and 9400 are virtually identical. It has been a few years, the last time I compared the company brochures, that is what I remember.
In the field, our experience has been the 9400 will outcut (some, not alot) a 7720, horsepower and newer (larger diameter) cylinder design being the primary contributing factors. We run a 1994 9400 until 1999 when we traded for 9410, and now have over 2000 hours on it. Excellent machines, both. So are the 7720s.
You are asking about a 7721, with the tractor as the variable. In my opinion, a 7721 (pull-type) will not outcut a 9400, unless it is an early 9400 with the 359 engine, which I have not had experience.
You did not ask about comparing a 9500. The primary difference between a 94xx and 95xx is the 95xx has 18 more inches of walker, and more HP. Otherwise, a 94xx and 95xx are identical. Just in case someone asks.
Back in the old days (1960s), my dad and uncle had a 65 (pull type version of a 55) and a 55. They pulled the 65 with a JD 630 or JD 3010. The 65 had its own power unit. The 55 always outcut the 65, maneuverability being better with the 55. We were cutting standing wheat in Kansas. If you are picking up, There may no be much difference.
Interesting question you posed.