Simple intervals are not bigger than an octave while compound intervals are larger than an octave. Ninths, tenths, elevenths and thirteenth are examples of compound intervals. Octaves, thirds, fifths are simple intervals.
Each compound interval is related to a simple interval. By subtracting 7 from the compound interval you get the related simple interval. For example 9 - 7 = 2, so a 9th is related to a 2nd. A compound interval has the same quality as the related simple interval. Several examples of compound intervals and the related simple interval:
See Reading Music in the Tutorials section.