Big batteries have much more sophisticated battery management systems that are designed to be as kind as possible to the cells. Also they reserve a capacity of 10% on either side, leaving 80% usable capacity available to the user. Li-Ion batteries don't like being fully charged or fully discharged.
Also due to their capacity most EVs are charged at about 0.1C whereas phones (especially with rapid chargers) are charged anywhere between 1C and 3C.
