Thanks!
In-character reason - half-breed saiyans are typically more powerful than pure-blooded saiyans, because they have better self-control due to exhibiting less rage and pride. Which isn't just me talking out my tailhole, it's also pretty well shown in the anime (discussion).
Out-of-character reasons - one, to put her on parity with the rest of the group without using the kaio-ken, and two, I assume the group will be a bit stronger once we reach the Meal arc.