Why and what does spotify upload when I switch back from offline mode?

[Retford]

Every time i switch back from offline mode spotify starts uploading. It uploads quite a lot, around 5 MB I think. And no, that isn't much compared to a lot of things, but i can't understand why spotify would need to upload any more than a few kbs. We've got a 5/0.5 connection split on 7 gamers here. When I go online on spotify, it maxes our upload speed for around a minute and everyone in the house get's disconnected for the duration. 

Does anyone know what spotify is uploading?