Does mastering make a song louder? It depends on three factors; 1) the style of music, 2) the quality of the mix and 3) what the producer or artists desire.
My philosophy is to preserve the dynamic range of the music wherever possible.
For example, patient if I am mastering a track with acoustic instrumentation that is full of volume peaks and valleys as well as multiple instrumentation changes, then the final master should and will reflect the integrity of those dynamics. Often times the overall volume will be perceived as louder because the analog mastering will add a depth and width to the song, but the reality is the peaks are often no louder than the original mix, only shaped differently.
On the other hand, if loudness is what the artist wants
then that can be arranged!!! The trick here is to push the volume of the mix as much as possible without compromising the sonic quality. The better the recording and mix, the louder it can be driven without signal degradation. With an excellent mix, the overall loudness can be quite high while retaining decent dynamic range and excellent sound quality.