Trampling with hot BBW ladies is probably the most painful situation for a BBW's slave. Even without heels, the hard soles of their shoes dig deep into the slave's skin and body. Most slave's will not even stand barefoot trampling because it's allready that painful with a big girl.
Most BBW ladies love trampling as it's without any doubt the most effective technique to break the slave's resistance. He can only obey without any resistance if he does not want his body to collapse under the weight of the lady. It's very satisfying for the lady to know she could easily end her slave's life by stepping on his neck or his skull with her full weight - she's undeniably in control of her slave's life and will.