I'm not sure weight has much to do with it. I could see being very overweight might cause issues. If the amount we need to take increases substantially, it could, I imagine- be more difficult to keep it down.
I think it may be more to do with how well we follow the protocol. People often fail because they didn't do that. For example- they didn't fast, they didn't take antiemetics, they didn't measure the SN/ water. They drank it with milk or alcohol. They didn't drink all of it. They were found early in the process and stopped.
Maybe how our digestive tract works may influence things but, I'm not sure we can be sure of anything going on there really.