"overrepresented" is a relative term. However, the simplest definition is this : take the percentage of racial group X in the population of the country in question. Compare that to the percentage of individuals belonging to said racial group in the school in question. Technically, a group is overrepresented if it appears more often in a school or other institution than the group is present in the population.
According to this
http://www.aamc.org/data/facts/applicantmatriculant/table9-fact2006to2009detmat-web.pdf
Asians are 39% of the medical school matriculants.
According to this
https://www.cia.gov/library/publications/the-world-factbook/geos/us.html Asians are 4.43% of the population in the United States.
Face facts. Asians are massively over-represented, by a factor of ten times.
Now, once you know the facts, we can discuss
why this is. Judging by average MCATs and GPAs, asians richly deserve their spots in medical school. Medical schools have to actively discriminate against Asians in order to keep them from being the entire class. (about 80% or 90% of the class would be Asian if the medical school went by GPA and MCAT and an individual of any given racial group had an equal probability of succeeding in an interview)
Journals on intelligence find that Asians as a group have higher IQs on average than do other groups. This is one theory. Others include cultural differences or perhaps something about Asians that causes them to do better in school even more than their higher IQ would suggest. For example, there's evidence that Asians have somewhat lower sex drives and secondary sexual characteristics than do other groups, which I suspect would help them to stay on track in school.