The i4 is minimally larger than a Model 3 on the outside. It's 3.7" longer, .1" wider, and .2" taller. On the inside, the Model 3 is roomier than the i4. The model 3 has a .8" longer wheel base, and more than double the cargo capacity with all the seats in place. For interior passenger space, the Model 3 has 2.1" more front head room, 1.2" more front leg room, 1.1" more rear head room, 1" more rear leg room, and .3" less rear shoulder room.
There might be a couple hundred pounds of extra interior bits and sound deadening, but most of the difference in weight is because the BMW needs larger batteries to achieve the same range as the Tesla. It simply isn't as efficient.