If the snapshot disk is a delta disk, then the size of the snapshot disk created should increase when going on working with the virtual machine ?
I could just give you the answer however read the previously linked KB as it answers that question and more! 
But the snapshot disk files from Snapshot-X created on Date-X a few months ago (with a size of 3 GB now) still have a file date value of: Date-X !
Yeah that certainly sounds right under the right conditions however I prefer to deal in technical facts so if you want proper analysis then provide the vmware.log files and a directory listing of the target VM showing filename, size and date/time stamps. Archive (compress) into a single .zip file and attach to a reply.