Is there like a standard that says that?

Naaah... just my personal bias. But before you consider a smaller speaker to be a true subwoofer, check out the specifications. Just because it can produce output at 30 Hz doesn't mean it is doing so at a useful level. The volume may be rolling off at 12--24 dB per octave, so that the really low frequencies while technically there will be virtually inaudible.

Larger speakers produce lower frequencies. (Well, Doh! Nothing like stating the obvious, right?) I can see where an 8" speaker would "round out the bass tones" but you might be missing out on the lowest frequencies.

tnstaafl.