There can be a number of different reasons for this. The most common one would be that you or someone else have installed something with a COP file, for example a device firmware or a device package on a node, but not on another. It can also be caused by that you or that elusive someone have added files to the TFTP directory(s), like background and/or MOH and/or annunciator, again on some but not all nodes. These are some examples of the most common reasons for why you might see different sizes for backup between nodes.
