Small stomache?

When I got my two kittens(now almost a year old) I free fed them and they grazed all day. The hit a growth spurt and stopped grazing and started eating everything that was put down so I eventually switched them to plan meals.

Now my newest addition(6 y/o) came to me overweight and was use to being free fed. She won't eat all her food in one sitting, which I thought was because she was use to being free fed. I am now thinking she has a very small stomach. She will only eat a lick us wet and a few kibbles of dry then walk away. Anytime I feed her anything she loves(different wet varieties) she eats a lot then some dry. Within 5 minutes she pukes it all up and won't eat again for the rest of the day. Same with treats. If I give her more then 1 at a time she pukes it all up.

The issue is she is losing a lot of weight(I don't own a scale but I can see it). She is not yet too thin but if this keeps up she will be. I have to feed her seperate from the other girls and keep offering food throughout the day. The problem is when I am away all day if I leave the food alone, the two kittens will finish her food as soon as she walks away from it, then she is hungry until someone gets home.

I'm not sure what to do. I can't keep her in a room by herself when we are away beacuse she gets too stressed and won't eat anyways. Any suggestions would be much appreciated.