The "Subreport" object has the "PrintOnParent" property which can be useful in some cases. This property is False by default.

Usual subreport is printed as a set of bands on the basic report page. In this case the parent band (which contains a "subreport" object) do not depends on the subreport bands, i.e. can't stretch. If the "PrintOnParent" property is True (you can set it from the object inspector or in the context menu), subreport's objects are printed physically on the band which contains the "subreport" object. You can make this band stretched and put on it stretched objects: