Karrin Murphy from The Dresden Files. Despite being five-nothing, blonde and adorable, she can chip a man's molar with one punch. There's nothing supernatural about her strength, though, as she's been studying aikido since she was eleven.
The Denarian Tessa fits this trope — when she's not a disgusting half-mantis demon.
The Fey Ladies, the youngest of the Three Queens, are pretty, young women with the third largest punch of their court, Summer or Winter. YMMV on Maeve, though.
Ivy, the Archive. Several members of the White Council are under the belief that she has the power equal to one of the Faerie Queens. It's heavily implied that they are severely underestimating her.
Warden Commander Luccio, the senior-most combat wizard in existence, considers Harry this. She also counts as this, once she is trapped in the body of a cute twenty-something co-ed.
Monster, a Hulk parody that showed up in the McGuiness issues of Batman/Superman comics. A cute eight year old girl that can transform into a muscle-bound brute at will. Batman is soundly horrified at the concept of taking her into combat; he might have recruited a few fairly young and inexperienced sidekicks himself, but at least they were all out of high school.
Luisa from Sho Shan Y La Dama Oscura. She's described as a 15 years old girl, with curly blonde hair, dressed in a Lolita fashion and even that she knows how to defend herself. Add the fact than when she was 4 years old she managed to escape from an hospital with her sister, attacking a lot of doctors on the process.
In the Mistborn trilogy, Vin, who is barely over five feet tall is one of the most powerful of the titular Mistborn, including the use of pewter which gives her super stamina and strength. This is often used to her advantage, as her opponents are less likely to see her as a threat.
Although, surprisingly, the drawbacks of being so petite are also commented on and even shown. Because Pewter scales with physical strength, Vin is nowhere near as strong as a person with comparable allomantic ability and a larger build. Also, her speed is negated if she is forced to grapple and her short stature also limits her reach.
Downplayed in Cassandra Kresnov. Sandy and other GIs are shorter than average, and Sandy explains that combat myomer doesn't handle well in large volumes - if she were a foot taller, she'd suffer chronic tension and be unable to control her own strength.
Urban Dragon Arkay is so petite she could pass for a teenager, or even a child. She could also punch through a brick wall if properly motivated.