Streaming platforms use “loudness normalization” to deliver a better listening experience to users. The goal of loudness normalization is to adjust the playback level of all material to the same average level. The result is a listening experience where all material sounds about the same level, so you never have to turn things up or down from song to song.
Any drop in volume applied by Spotify is a simple drop in gain, and is entirely harmless.
Spotify offers more in-depth information about this here: https://artists.spotify.com/en/help/article/loudness-normalization
That said, if a Spotify Premium user has selected Spotify Loud for their playback, anything coming in quieter than their normalized volume gets a limiter applied to it.
In the end, we recommend mixing your music so that it sounds best to your ears in terms of dynamics and frequency balance.
Don’t compromise your sound with streaming platforms’ playback volume in mind! A mix with a healthy dynamic range and frequency balance will sound best at any volume.
Here’s some advice and best practices from one of our resident engineers: