Audio signal dynamics and musical dynamics are different things.

There are certain cues we expect when interpreting whether something is 'loud' or not. One is the general (but not exclusive) rule that louder sounds in nature tend to have more upper harmonic intensity. Another is compression which mimics the tympanic reflex in our ears. A third is dynamic reference - introducing sounds that are quiet will place the louder sounds in perspective and make them seem more intense.

The fact is that if the meters are reading 0dB then in terms of the audio signal its not 'quieter' than a commercially mastered song, its that your ears are interpreting it to be quieter because of the quality of the sound. You have to introduce cues that a listener can identify as 'loud'.