As with any website, when you upload videos and audio to YouTube, they all shrink. This is done in order to reduce the file size due to the loss of not the most important components. However, this is not a YouTube issue in itself. What’s worse is that since about 2015, the site has started lowering the volume in music. The video service adjusts the volume so that each video on the site sounds about the same.
After all the processing, the songs on YouTube become more muted and not quite as originally intended. You can try to compare tracks by listening to them on different services.