Out of the 15 hamsters I've had in my life, 5 of them were eaten by my kitties.
This was because of the cage structure. If the fat cat went and sat on the cage (wire or plastic) it would either have dent or break some part of it. And it would allow the smaller cat to reach in and get the little hammy.
Cage structure is pretty important now that you have a cat. Because like Madpiano said, it's like "cat tv". It really is. It's like watching really good restuarant buffet commercials for them, except with real food that moves.
If you have tubes, the kind that come out of the cage...you might be in trouble. They break off pretty easily. Anyhow, I dont think there is anything you can do to stop this behavior. It's natural. Just make sure your cage is sturdy, and not on something that it can be easily pushed off. (Lost 2 fish to kitties that way)
If you want to get really into it...I would suggest getting the kitty repellent spray and spraying that just by the cage, not in the cage obviously, but around it. Or lemon juice all around the cage. Cats hate the smell of lemon juice. Otherwise, again like Madpiano said, hamsters are pretty good defending themselves, and your kitty might learn soon enough how close he can get to the hamster.