Well, i got to thinking about it but i don't know the numbers. If a 5x5 patch of cursed earth spawns faster than the grinder can kill, then it would explain Bevo's patch of 3x3.
It is 5x5 and the grinder can keep up with the cursed earth well.
All my grinders use 5x5 even those in my video linked above. In the video above I have 3 cursed earth rooms and each of those cursed earth rooms are 5x5.
[EDIT:] I should correct myself. In a recent SSP LP I did use 3 grinders per 7x7 MFR Spawn room, though all of my cursed earth rooms were less than 5x5 SSP LP build. Why the cursed earth rooms in there were less than 5x5 was I miss counted when doing the build and didn't feel like redoing the entire main building to make room for 4 5x5 rooms so just left out the walls. lol
Regarding the 7x7 rooms there were several reasons for the 3 grinders per... One I wanted to try out 7x7 rooms since I was using MFR Spawners for ghast. I wanted to test for better yields. Why I used 3 grinders per room is the grinder only do 5x5 and the rooms were 7x7, also MFR Spawners are crazy fast and I wanted to make sure I kept up fast. So ghast didn't sit in the room for long.
Note: the yields on the 7x7 Spawn rooms was bloody nuts. I mean the ghast and witch rooms were producing just crazy amounts of stuff. They were wild. From my test the yields were far better.