9. why do atoms have relatively the same size even though they may have widely different masses?

They don't have relatively the same size.  The most massive atoms,
like Uranium, are much SMALLER than the least massive ones, like
Hydrogen !

That's because the most massive atoms have so many more protons ...
positively charged ... in their nucleii, pulling so much harder on the cloud
of electrons ... negatively charged.
 

An atom's smaller negative particles are at a distance from the central positive particles, so the negative particles are easier to move.