@jerv had it right. There are scaling factors at work that totally change everything. The difference between an ant and a man is at least two orders of magnitude.
A cow-sized ant would collapse under its own weight, would overheat from its own metabolism, etc. The whole scenario becomes preposterous if examined critically.
Likewise an ant-sized man would be torn apart by surface tension of water & would be unable to maintain normal body temperature. @ragingloli and @thorninmud explained other difficulties with inadequate respiratory, circulatory, nervous, & digestive systems. On the plus side, a person could lift many times her own weight and jump many body lengths in a single bound.
The real question is, do atoms stay normal size or do they reduced proportionately? If they stay the same size then we can confidently predict physical behavior & the complexity of the human body is unattainable.
If atoms shrink as well, however, then we first have to re-write the laws of physics and chemistry before even answering the question!
There are many fictional works that imagine people shrinking to microscopic size, or bugs becoming giant monsters, but they never realistically confront physical theory.