So, your co-worker, since she works full-time, gets an actual lunch hour.
Does she eat her lunch at her desk or does she go elsewhere to eat?
Since you are part-time, you don't get a lunch hour, but you do get a 15 minute break. Do you eat your lunch during your 15 minute break away from your desk or do you eat your lunch at your desk?
Are you both sitting at your desks eating lunch at the same time? Does your supervisor come in w/ lunch for your coworker & your coworker eats it at her desk in front of you?
Does your supervisor think you're eating your lunch or snack or whatever during your 15 minute break? Does your supervisor assume, since you only have a 15 minute break, that you don't really have time to eat a full lunch?
Are you allowed to eat at your desk?
I think the details matter.
If your coworker is going elsewhere to eat her lunch during her regular lunch hour, it would matter less to me. She gets a lunch hour, & you, since you're part-time, don't. Additionally, since your co-worker is full-time, she is probably performing additional duties & there may be instances where she goes "above & beyond" & this is the way the supervisor is showing appreciation.
However, if the both of you regularly eat lunch at your desk & your supervisor is bringing in lunch for your co-worker & she eats the "gifted" lunch in front of you while at her desk while you are also eating your lunch at your desk, that's a little bit different.
It would be nice if your supervisor brought in a lunch occasionally for you or maybe some other kind of treat at the same time she brings in lunch for your co-worker - a cup of coffee, a cookie, a milk shake, etc. And, depending on how & where the lunches were being eaten, I might be bothered. However, if the supervisor is bringing in lunch for your co-worker, who has an actual lunch hour, & the co-worker is going else where to eat her lunch (away from her desk), I'd be less bothered.