Does your watch show more than 8 GB synced? Mine does not.
There's 32 GB of storage, sync uses 8, and it is really hard to fill more than other 8 with OS, apps & data.
> Apple Watch limits the amount of music which can be stored on the watch and is typically limited to 24% of the available space on the watch, so given you have a watch with 32GB of total space, 8GB would the maximum you can store.
I use nearly all of those GBs of storage on mine with synced music and audiobooks for running.