Yup, the expandable BMW bag is reasonably large. The positive bit is that the expandable one holds a day's worth of kit - shoes, clothes, toiletries, leccy gubbins. In my case that's what comes into the hotel. Everything else stays in the panniers (spare clothes, waterproofs, locks, tools, etc).
It's overkill for wallet & phone, but then again, 'What'll hold a lot will hold a little'.