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.

You need to be more specific.  i really don't know what a "motion map" is.you can draw a graph of an object's position, distance, displacement, speed,velocity, or acceleration versus time, or you can draw a graph of the object's x-position versus its y-position and not give any information about time at all.if the graph presents position or displacement against time,  then constant velocity is a straight line . . i.e. a line that has  constant slope.if the graph shows speed against time, then constant velocityis a horizontal piece of line.if the graph shows acceleration against time, then constant  velocity is a piece of line that lies on the x-axis . . it's value is zero.