A very small population of a species faces a greater threat of extinction than a larger population. Smaller population faces more random fluctuation, which may result in greater death rate than birth rate and that ultimately leads to their extinction. Few reasons are:
Small population promotes inbreeding. Inbreed results in lesser variations.
A small population is vulnerable to sudden change in environment. Because of genetic drift chances of species with small population may wipe out.
This lead to negative population growth which is detrimental for the survival of the species with smaller population