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I am having problems reprojecting items in ArcMap 10.1 from British National Grid To WGS84. (WKID: 27700 Authority: EPSG to WKID: 102100 Authority: ESRI)
All Polygon Layers I project [I use Project (Data Management)] from BNG to WGS are basically twice as big in metres as they were originally. Even the Shape Area and Shape Length inside the attribute table changes.
ESRI:102100 isn't WGS84 (EPSG:4326), but "WGS 1984 Web Mercator". The Mercator projection is conformal (maintains shapes) just like tranverse Mercator which is what British National Grid uses. However, it's generic for the entire world, while BNG's parameters have been customized for Britain, Scotland, and Wales to minimize distortions.
I am not surprised that the distances and areas are different. Remember that in the Mercator projection, the longitude lines are straight, vertical lines. In transverse Mercator, it tries to model the longitude lines converging to the poles. At 51.5N, East-West distances are very roughly about 0.6 the distance at the equator. Mercator doesn't show that.
Many ArcGIS tools use 2D calculations when the data is in a projected CRS, so I believe that's what's happening. In ArcMap's Measure Tool, try changing its distance calculation from linear/cartesian to geodesic as a check.
