Yes, there is an intervention you can do. Also, I want to commend you on your intuitive insight that Sunny's size and behavior could have something to do with being bullied away from food.
I wrote this article
https://www.backyardchickens.com/articles/chicken-bully-chicken-victim-a-two-sided-issue.73923/ because this is such a common problem that can have chicken keepers frustrated and stumped as to how to deal with it. This method I came up with has had 100% success every time I have used it, the most recent being just this month.
I'll tell you about it so you can see how well it works to treat the victim of the bullying while not worrying too much about the bullies.
Ethel is a three-year old EE. She's high strung by temperament, but she's been a basket case since she started a very hard molt. I first noticed something was wrong when I was seeing runny green poop in the morning on the poop board where she roosts.
I brought the problem to the BYC community and we tossed around ideas, but during one visit to the run to check on the chickens and dish out some food, I noticed Ethel going up to the food but when another chicken walked up, she ran away without eating. I stuck around and watched for a while. The same thing happened again and yet again.
I got a dish of food and put Ethel in the "jail" isolation pen in one corner of the run. She wouldn't eat right away, but once the other chickens lost interest in why she was in the isolation pen with her own dish of food, Ethel tore into it not coming up for air for a solid fifteen minutes. That told me all I needed to know. Ethel had been starving.
So Ethel got two days in the safe pen, never leaving the flock, but she had peace and safety and all the access she needed to food and water so she was able to regain her strength. I've seen before how a chicken that is weak will also be withdrawn and timid. It doesn't take long to build back up a malnourished chicken. Ethel herself signaled to me she was ready to go back into the flock with the others, and she's been just fine ever since, getting all the food she needs, and even trampling others to get at treats first, snatching the food out of other beaks when she feels she deserves it more.
The key to the success of this system for rehabilitating a withdrawn chicken is to keep them in the flock but protected so they can relax and trust themselves again. The first time I isolated a timid hen that was being severely bullied, I couldn't believe the personality change that took place. When isolated but still able to feel part of the flock, a timid chicken is able to recover their self confidence and compete normally again for necessities.