Loudness is not the same as amplitude or SPL. Human ears are more sensitive to midrange frequencies (2–5 kHz) than bass or extreme highs. A bass-heavy track at the same SPL will sound quieter than a midrange-heavy track. LUFS (Loudness Units Full Scale) is the standardized measurement that accounts for human hearing sensitivity.
Streaming platforms normalize audio to consistent LUFS levels: Spotify −14 LUFS, YouTube −14 LUFS, Apple Music −16 LUFS. Mastering hotter than the target does not make your track louder on streaming — it just gets turned down, potentially with an audible limiter.