If the size range for the cluster size is the same for the minimum and maximum, the hosted layer's maximum symbol is displayed as small in ArcGIS Pro.

Workaround

1. In ArcGIS Pro, right-click the selection layer in the Contents pane and click Symbology.
2. On the Feature Layer tab, in the Drawing group, click the Aggregation drop-down menu and choose Clustering.
3. In the Symbology pane, on the Primary symbology tab, ensure the minimum and maximum sizes of the symbol are different from each other.

Steps to Reproduce