Could gravity be much stronger (or weaker) at the atomic scale? If gravity is mediated by particles and you are at a scale where those particles are relatively much larger does that perhaps imply that gravity can't work exactly the same way at very small scales as it does at much larger (like planetary, galactic) scales?
 A: Regarding the broad question which you bring up, yes, it is possible for gravity to have a different strength from expected at extremely small scales. But it doesn't have to do with the size of particles. It involves what's called the ADD model, or the theory of "large extra dimensions" ("large" being a bit of a misnomer). It postulates that our universe may have hidden extra dimensions which we never notice because they're very small relative to our everyday concepts of distance.
But at extremely small scales, these extra dimensions would become very important, changing the relationship between distance and the strength of gravity (The inverse-square law would no longer apply.). At these scales, we would find that gravity is much stronger than expected. In fact, this theory was proposed as a possible explanation for why gravity appears to be so much weaker than the other fundamental forces (the so-called "hierarchy problem"). It's so much weaker because much of the gravitational force essentially "bleeds out" into this hidden higher-dimensional space.
