She was too chunky for that role

You know, I normally don't like to body shame, but I agree somewhat. Her size is fine IMHO, but if she had just toned her body in order to look like she actually spent half her life training how to fight to the death, I would've believed it. Ronda Rousey (I don't know much about fighters, so she's the only name that comes up) isn't exactly stick thin, but she's a trained fighter.
I don't think the size has anything to do with it, but the muscle definition definitely does.

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Never forget what you are, the rest of the world will not.

shuuuuuuuuuut uuuuuuuuuup

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Love means never having to say you're ugly. - The Abominable Dr. Phibes

Her physical fitness isn't what bothers me the most. Her confidence comes across as too soft for an assassin. The little girl in the beginning lacked the empathy of an assassin which drew me in. I was thinking this would be like Briggette Fonda's Point of No Return, but instead this older girl comes in looking and feeling like she was sent to a convent for ten years.

I thought that from the very beginning and throughout the whole movie. Someone trained for 12 years would be pretty cut up. Not to mention when she took a fighting stance she looked like she was putting her fists up for the first time ever.

I agree and before reading this post I wrote how I felt Abigal was not the right person for Veronica.

I like Breslin, but feel that her appearance and body language were entirely wrong/unconvincing for the role she played. Even if her character was written as having vulnerabilities and a lower level of ruthlessness to make her more likeable to the audience, the unprepared state of her body and fighting abilities completely undermined the notion that she had spent years acquiring the skills and mental focus to put an end to the gang.

To be fair, Breslin was sabotaged by the incompatible and unequally balanced facets of the script/production. More emphasis was placed on how pretty/attractive she looked than on making her a convincing assassin.