It's probably the EQ settings. Do you have immersive sound enabled on your 3? You might want to try that as well.I felt it sounded more full than mine. When I go back to pick my car up I'll try and do a side by side playing the same song. It may have also been the eq settings being different between cars. But I found the low frequency range from the front speakers sounded more full in the S than on my 3.
To be honest, I've never really been super impressed with the sound in the 3. But when I got into the S I felt immediately that the sound was more immersive.
Could just be a placebo effect, or to do with the cabin geometry, or EQ settings, or maybe it is a better sound system.