Would a 21 hp Kubota 7510 be big enouph to put in food plots?

Yes, you would just need to size your implements to the tractor and soil type. I use mostly an ATV and have little trouble.

It would be great for food plots. At one of our places i plant 4-6 acres of food plots twice a year using a Mitsubishi 18 horsepower tractor.

It will work fine, but after a while you will want a bigger one!