Thank you for your question! As a balloon inflates, the air pressure (or helium pressure) inside the balloon changes. It can be theoretically and experimentally shown that the pressure increases until the balloon radius is about 1.4x the uninflated radius. Above that point, the pressure continually decreases until eventually the balloon pops. The wikipedia article on the "Two-balloon experiment" shows this. It's very interesting!

Let's assume the balloon is inflated at constant temperature. Using the ideal gas law, we can see the that density of air molecules (N/V) is directly proportional to the air pressure. This means that as the pressure increases, the air molecules are closer to each other, and attract each other more strongly due to van der Waals interactions. Because the pressure inside the balloon is typically not very large (about 1% greater than atmospheric), these van der Waals interactions are mostly unaffected during inflation. Thus, the largest difference molecularly is simply the density of air (or helium) molecules.