I know.
But my problem is different:
I insert a circle (radius = 1000 meters in EPSG:3857 Web Mercator, in Meters);
after I transform it in WGS84 (in degrees);
In ArcMap by a tool I measure the radius (the tool returns length in Meters, etc...) and radius is 700 meters, not 1000 m.
I think it's an issue of SDE.ST_Transform