how do you feed two cats different amounts?

Our cats are both quite different in weight
Bobbi is about 5kg
Zooey is about 3kg

how do we make sure each cat is eating the right amount?
i don't like the thought of feeding once a day
what if they get hungry?
so we leave food out for them and let them sort of graze
BUT
the problem is that bobbi grazes a bit too much and as a result is a little on the chubby side
nothing serious
but i think we'd better nip it in the bud

how do we do it?
any ideas?

Here's what I wrote to Bloomp a couple days ago. .. I don't know if the weight disparities of your kitties warrant a Zooey-O-Mat

Easy. We made a Beck-o-Mat (skinny kitty's name is Becky). Get a large box. Cut a little hole at cat level that ONLY Noodle can get in and out of. Feed Noodle in here. Train Noodle to get in and out. Wallah! A Noodle-o-Mat.

It may take a couple test runs to get the size of the hole right. Once you get the proper size hole, you can get fancy. . . make one in a cupboard, or any other kind of enclosure in your house.

You also want to weight this box down, and place it in a corner because the larger cat will be trying to get into the source of the food and will be pushing it all over the room.

I would also see if you could get your larger cat more involved in exercise, even harness training and taking the cat for a walk.