Hmm, well if you have an image that is 4000x4000 (for example) WordPress will create the alternate thumb, med and large size and appends the size to the filename - which is where you see those "1024" images. They are not considered a "post" on their own and they don't get a unique record - so Imsanity never even sees that they exist.
If you upload an image that is lower than 1024, though, WordPress won't bother to create the large size because it's bigger than the original.
So, perhaps if you bulk resize images so they are lower than the threshold for WordPress "large" size - internally WordPress is doing something now, thinking that the large image size shouldn't be there or something. I'll have to test that. I haven't seen it before, though.
Good forethought making a backup - I definitely recommend it before any bulk operation!