It could be various things causing that including the media player settings. What you want to do is mixdown in Studio One and either check "Import to Track" in the dialog so it will come back into the song, or drag it back in manually. Then....
Assign it to a sub out using the same outputs as the main, to bypass any master bus Fx, and solo it and A/B it against the multitrack mix. If it sounds the same volume wise, it is what it is and any difference you hear in any other media player is from another influence. If not, if it sounds lower there, it may be that it didn't mix correctly or you are hearing things from the multitrack mix that are doubled somehow that aren't getting printed on the mix, making it sound louder.
The key is to compare it directly to the main mix in Studio One. After that, you'll know if it's the same or not.