Why are small populations more prone to genetic disease?
Gene pool is small on small populations. So probability to gather disease causing genes is very high.
Diseased individual number is low, generally. It is beacuse of natural selection. Diseased ones cannot successfully reporuduce beacuse of that ıt genes are transferred less to future. It means they are eliminated.
Most of diseases are recessive. So recessive individual number is very low, because of natural selection mentioned above. In big populations, match between recessive indviduals has low probability while in small population, this probablity is being increased.