Not sure if there is a way to or not, but if your feeling techy some googling is ahead of you. Your iTunes backups of your iphone are stored in some folder in AppData. From what i've seen, and if i'm correct in remembering here... most of the files in that backup have cryptic names which are seemingly random strings of letters and numbers. Those files contain various data from your backups. There is a file in there that keeps track of all those files and what they are for. You need to find this program (which I can't remember it being called) that will actually take that file, and parse it out and tell you each of those cryptically named files is for. (one of those files is the creepy stalker database file that tracks GPS location) So with that you just see what files are being used for those large video backups and delete them.
That's how I would approach this anyways. Lord knows I would do that and then the file that keeps track of the other files would throws up and itunes would spit errors like nobodies business...
Good luck, happy googling! Hopefully someone a bit more knowledgeable about itunes helps you out :)