Yes, you need to take it and yes you need to apologise sincerely and here’s why.
She looks better now, right? Know why that is? Because rather than be able to accept herself, she has been forced into a place where she is afraid to look a little overweight. She might look great for the rest of her life but it will be through fear - fear that you caused.
People who are bullied change and nearly every aspect of it is self-destructive. So from being a girl who was a bit overweight and had a few things that a bullying idiot could pick up on as an easy target, she is now determined never to go through the pain and humilation of being laughed at just for being her. So, externally, she will do everything in her power never to be that easy target again.
She is rude to you now? Well, that’s because she has never recovered from being victimised by you. Every time she sees your face or hears your name all that pain comes righ back into sharp focus.
She is rude to you, probably more as a way of shielding herself from that weak victim inside her so she tries to throw a few barbed comments at you as much to try to feel strong against her attacker as to get any form of payback. Frankly, I’m guessing all she wants is for it never to have happened - but that doesn’t work.
So, whatever she throws at you take it, not as payback or Karma but as the manifestation of a situation you created. When she is a mature woman, she will still be trying to lock away those memories and they will still come back to haunt her when a little bit of ribbing from colleagues or friends hurts that bit more than it should and she can’t handle it.
If you apologise, make sure you do it right and don’t expect to be her friend. Make sure your apology contains only your failings - your fear and the cowardly way that you dealt with that fear.
And maintain that standpoint when she asks you why you picked on her for the incessant barrage of abuse you directed at her.
Then learn from it.
The good news is that we all make mistakes when we are younger - big ones. You don’t have to spend the rest of your life beating yourself up about it if you learn the lesson and treat people with respect in future.
But don’t try to forget it, either. Remember it as a time when you realisied that things can never be undone.