Billboard sizeInMeters not accurate in Columbus View with WebMercatorProjection

I just noticed if you set a Billboard with sizeInMeters to true in Columbus View with WebMercatorProjection the size in meters is not accurate.

Bellow are screenshots from the sandcastle of AGIs parking lot where each space is about 6m x 3m. The image from the columbus view is almost 1m shorter.

Not sure if this is the expected behaviour with WebMercatorProjection?

[Sandcastle example SCENE3D](https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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)

[Sandcastle example COLUMBUS_VIEW](https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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)

Browser: Any

Operating System: Any

Start by reproducing the linked Sandcastle examples in 3D and Columbus View with WebMercatorProjection, using the parking-space dimensions as the reference. Compare the Billboard sizeInMeters result between views and determine whether the discrepancy is a rendering bug or expected projection behavior; done when the behavior is corrected or clearly documented.

Written by the indexing model from the issue text.