Why do some songs play louder than others on my android?Even the same song

[BiggyPhilz]

When I add music to my playlists on YouTube using (in particular) I notice some songs play much louder than others. Even down to the same exact song... What's the problem?

 

[B. Diddy]

Welcome to Android Central! Do you mean that the volume is clearly fluctuating throughout playback, or that each song has a much different constant volume?