The Tesla Model S is great but it is already a few years old now so it is only normal for the Tesla Model 3 to come with better features and tech.
It is believed that the upcoming Tesla Model 3 could be coming in with a battery pack that is way smaller than what the Tesla Model S and Model X will be offering. The battery might be smaller but it will not affect the electric miles of the Model 3.
The trick here is to increase the energy density of the battery pack. That means they can now fit in more energy into a battery pack and that only reduces the size of the battery but it also cost less since fewer materials are needed to make these batteries now.
It is believed that as Tesla keep working on improving the battery, their battery will only get smaller and smaller.