The answer to your question lies in the number of people who are buying the MagSafe battery pack. If it is a large amount of all iPhone customers, the phone should be just thicker with a bigger battery, if it is a small fraction of all customers, the battery is probably fine except for some high workloads. Personally, I am fine with the battery capacity (iPhone Pro Max), I wouldn't mind a little bit more of thickness for a bit more reserve, but it shouldn't get much thicker.