the bbox (is that how you call the RoD) is adjusted, but not the Format if you set the size manually, because it can be animated (and the format can not)
is there something unclear in the doc?
Reformat / reformat Boolean Off Translates the bottom left corner of the crop rectangle to be in (0,0). This sets the output format only if ‘Format’ or ‘Project’ is selected as the output Extend. In order to actually change the format of this image stream for other Extent choices, feed the output of this node to a either a NoOp node which sets the proper format, or a Reformat node with the same extent and with ‘Resize Type’ set to None and ‘Center’ unchecked. The reason is that the Crop size may be animated, but the output format can not be animated.