I finally waited. The flower was indeed larger than the plant itself – well, about one and a half times larger, to be sure. It opened in the morning, and even exuded a faintly sweet smell.
Unlike Gymnocalycium anisitsii, which bloomed for a week or more, the lobivia flower began to fade on the second day. So I was also very lucky that all the beauty came on a day off and I didn’t miss it.