Why acid can break down molecules?

An acid contains H+, hydrogen ions. What makes H+ so special? H+ is very small, much smaller than any other ions and atoms.  Why size is so important?

Molecules are atoms grouped together. The nucleus of atoms are positively charged. They repel each other. These atoms can stay together because of the negatively charges electrons. These electrons attract nucleus and glue these nucleus together. Don’t negatively charged electrons repel each other? Well, electrons are much smaller than nucleus. They can evade clash much easier. 

H+ are very small. They can wedge into two nucleus easily. All nucleus are positively charged and repel each other. The increase of repellent power helps break down molecules. This is why acid can break down many molecules.

How much is  H+ smaller than other molecules and ions? H+ is the only ion without electrons orbiting around the nucleus. Imagine the size of the sun and the size of the solar system, which includes all the planets orbiting around the sun. Now we can see the vast difference between the sizes of H+ and other ions. With size in mind, we can appreciate the ability of H+ to wedge between different nucleus and to break down molecules.