The final mastering/mixing has an avg db level they target. The more compressed mixes tend to have much louder avg db. Some of these modern "loud" mixes target -3db whereas the previous ones could be -10db or something. Played at an identical volume, the more compressed track will sound louder due to this target. Loudness war is an apt name. Just look at the sample waveforms on the wikipedia page:
* https://en.wikipedia.org/wiki/Loudness_war