If you selected a content search, then you are correct, the files will always be the same size.
However, Duplicate Cleaner can also match by other critera, such as same filename, audio tags, or a similar image. In these cases the file sizes may well be different (eg. same picture, at two different sizes), but they are 'Duplicates' according to your search critera.
I just downloaded the trial for Pro and have been using the free version for awhile. This is an older post, but I found it interesting because of something I noticed and didn't know the reason behind. Sometimes I save an image file (usually tiff) to a different name before I work on it. The file should be identical, but in my Photo Viewer (Bridge) the resaved file is always a little larger. Is something added in saving the file? Would the hash values be different? The creation date would be different, but I'm not sure how that would change the file size.
My main concern is whether this difference will make it difficult to tell if the duplicate is a resaved file or has had a small retouch. I try to add -RT to such files, but sometimes forget.
I could have asked this as a new topic but it fit this topic subject perfectly.
By the way, I find that the free version often misses duplicates and am hoping the Pro version will find those.
