Sub-Report component can be placed on data band. When rendering the report, the Sub-Report will be rendered as an item in the databand, so this component will be printed in each databand. The figure below shows how the sub-report is rendered when the Sub-Report component is placed in the data band.

In this case, the height of the component on the sub-report page of the report will be higher than the height of the Sub-Report component. So if you put the Sub Report component in the databand and render it as an item in the databand, in this case the CanGrow property works and the component can grow by its height.