Why did the girl smile bigger and keep staring at me?

I had a cap on my head and wore big clothes because I lost a lot of weight, so my jacket was really loose. My face was red and didn't look great today, but despite that, she saw me and her smile still got bigger.

When I stepped onto the bus, there was a girl who couldn’t take her eyes off me. I looked in her direction but then glanced outside. I thought she was smiling at me, so I gave her a friendly, harmless smile. Her smile grew even bigger, and she looked at me again with her friend. When she and her friend got off the bus, they both smiled at me. I noticed she had a nice outfit, but her friend saw me looking and seemed to acknowledge it with a knowing look. I felt a bit embarrassed.
Why did the girl smile bigger and keep staring at me?
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