Remind your gf that clothes will often shrink in the wash or after being drycleaned.
Hormonal changes and water retention levels also affect weight along with bloating.
Ultimately there is little that you can do beyond reassuring her that you think she has always been in good shape, you remain attracted to her and encouraging her to stay active & healthy.
I do understand some of how your gf feels.
Sometimes an intense focus upon weight or other aspect of personal appearance can arise from: childhood bullying incidents, past traumas, trying to retain a sense of control or purpose, self-esteem issues or overcompensating for once being a plump child/visibly overweight adult.
I am a calorie counter and remain the same weight as when I was 13.
It would be a nightmare for me if I couldn't fit into most of my clothes because I had gained a lot of muscle or got visibly fatter.
You can also ask your gf whether she wants to change some of her lifestyle habits.
In terms of doing some daily youtube workouts, joining a gym or making some alterations to her diet.
If she wants to visibly lose weight, she'll have to go into calorie deficit by at least 500 calories a day.
Increasing her daily exercise and reducing her daily calorie intake from food, drinks, sauces and snacks.
She can also try doing some short youtube workouts that can help with toning muscles, posture and improving body appearance.
Weight loss cannot be spot targeted without surgery, everybody is different in terms of their individual fat distribution and places where weight gain or loss is most visible.