Amazon is a bit obtuse about this. Except for the quadruple cluster types which have specific info every other instance type is rated "I/O Performance: Low | Moderate | High" with no qualification for what low, moderate or high mean.
It is also thought that the m1.small has more flexible resource allocation than the larger types. There is a very early post from Dave on this (more than 4 years old) to that point. I do not know how current that info is, but customers do occasionally complain about m1.small having less throughput or memory than expected.