Twenty plus years ago when selling a kubota with a 60 or 72" belly deck and at that time 21 horsepower was what was offered for the 72" people would not believe that you could run it!
Many choose for the extra horsepower today for the extra speed.
When we sold the older cadets with a 48" deck and they were only a 10 or 12 horsepower unit you could slip the clutch to get them going. Today with the quickness of the electric PTO clutch it takes a minimum of about 18 horsepower to just start them although they still only need the 12 horsepower to run them for the most part.
The diesels especially and water cooled gas engines all have higher torque rise then a air cooled engine diesel or gas.