Why do atoms have relatively the same size even though they may have widely different masses?

The size of the atoms does not directly correlate its mass. The mass could be heavy or light, but it may appear to be about the same size. This is because the nucleus of the atom is very dense. Majority of the mass is inside the nucleus of an atom which occupies very little space. So, even though the element is very heavy, when it is crammed into the same little space, they would look like they are of the same size.