Most of these stories are from the time after Alexander VI's death, and were passed around by his enemies. If you step back and actually consider the nonesense of some of what is written about him (how is it physically possible for his body to "to swell so much that it became as wide as it was long", according to Burchard, who is so trusted by the slanderers of this Pope?). When one wishes to believe anything it is quite easy to manufacture all manner of spurious tales.
You've never heard of a 60 inch waistline?