【解决办法】设置billboard的sizeInMeters属性为true即可，参考代码：
billboard: {
image: defaultUrl,
horizontalOrigin: Cesium.HorizontalOrigin.CENTER,
verticalOrigin: Cesium.VerticalOrigin.CENTER,
scale: 3,
sizeInMeters: true,
// scaleByDistance: new Cesium.NearFarScalar(1000, 3, 100000, 3),
width: 50,
height: 50,
distanceDisplayCondition: new Cesium.DistanceDisplayCondition(0.0, 800.0),
heightReference: Cesium.HeightReference.CLAMP_TO_GROUND
},
