When resizing, MacDraw "remembers" the non-rotated position of each object in the group, and resizes each object based on its non-rotated state. For example, if you resize a group to widen it, and the group contains an object that's rotated 90 degrees, the rotated object will become taller rather than wider.
One option is to save out the group as a PICT or PICT2 file, then open it back into MacDraw. This object will then resize as expected. Another option in MacDraw Pro is to save as an EPSF file, although the objects in an EPSF file cannot be edited.