Whether a camera can have its resolution changed depends upon the kind of camera and the kind of interface you are using to it. For example there are customizations that can be done with gige interfaces that cannot be done with webcam()
In some cases, some webcams are only available at full resolution if you use a high enough speed port, and even in some cases only if you use a direct port and not a passive hub -- that is, in some cases webcams detect that they do not have a full speed connection and in that case only make a lower resolution image available.
