The creators explained their choices with the character.

Avatar: The Legend of Korra introduced us to Korra, Aang’s successor in the Avatar Cycle. A lot was riding on the show, given that it was being released at the heels of The Last Airbender, a live-action adaptation of the original animated series by M. Night Shyamalan. It proved to be a critical and commercial failure, essentially stalling any hopes the franchise had of making it big. The creators of the show had a very specific idea about the show in their minds, and they wanted to see it realized properly.

One of the many things that were important for the creators of the show, was that Korra should look like a warrior, someone who has trained their entire life in martial arts. The sole purpose of the character was to practice her martial arts and hone her skills, which would have yielded a muscular and toned figure for anyone. It was for this very reason Bryan Konietzko and Michael DiMartino wanted Korra to appear as muscular as she did.

The creators of Avatar: The Legend of Korra were against her looking like a ‘waif’

The creators of the show imagined Korra as a headstrong character, one who is totally opposite to the previous protagonist of the show. With regards to the character, Bryan Konietzko said:

“She’s muscular, and we like that. It’s definitely better than being a waif about to pass out. I know, I look like a waif — who am I to judge?”

It is immensely important that Korra looks like she is the Avatar. Even in the original series, Konietzko and Michael DiMartino did not shy away from depicting imposing female Avatars, most notably Kyoshi. While Aang was just 12 years old at the beginning of the series, the show saw him pick up some muscle mass during his training too, given the grueling regimen he went through to learn earthbending and firebending.

It makes sense for the world of Avatar to have the titular characters have a significant amount of muscle mass, given the martial arts training they have to go through from a very young age while being bombarded by threats from all sides.

Korra and the Earth Avatar will have to go through some brutal training

Avatar Korra found herself in a situation that no Avatar has found themselves in since Avatar Wan. Stripped of her power to access the wisdom of her predecessors, the Avatar Cycle has begun anew with Korra, which means the Earth Avatar who comes after her will have a tremendous amount to figure out. Korra has left notes for the Avatar (as depicted in supplementary works) after her, but this does not mean the Earth Avatar is well covered, knowledge-wise.

In the same book, Tenzin points out that the Earth Avatar might not have a connection to his past lives at all, prompting Korra to write down her accomplishments. This does not fair well for Korra’s successor, as not having the memories of past lives would be a massive blow to the Avatar Cycle, seriously crippling their power going forward.