Query to inform Rewind of times that a file was suddenly decreased in size.

I have a process that builds a time record to a file on a mounted drive. Due to concurrency issues that Dropbox does not manage I have another process that copies updates from the mounted drive to a Dropbox folder. 

 

Though the mount works 99% of the time, there are times on the road when the mount fails so that the process puts the time record to the mount point rather than to the mounted drive. Then the update process updates Dropbox with a truncated record. 

 

I want to look back over the rewind record to identify those moments where the Dropbox version of the time record file is suddenly reduced in size. These would be points where I want to recover the Dropbox previous version i.e. prior to the truncation. 

 

How can I do this? 

 

To clarify with an example :

 

Jan 1 save.dat is 1 M

Feb 1 save.dat suddenly becomes 10 K

March 1 save.dat has built itself up to 750 K

April 1 save.dat is 20 K

 

I want to be able to merge all these versions. 

I don't expect Dropbox to perform the merge, but I want to perform downloads of each version at its maximum size.