Just because Aurora was slender, didn't mean she was healthy. It was likely that Tonks could beat her in a foot race any day, pregnant or not, because it was ridiculous how good of a point Tonks made when she said Aurora should go running. Because yes, it wasn't healthy to spend all that time up in the Astronomy Tower. But good luck to anyone that tried to get her to say that. "Do pregnancy hormones make one delusional? I wonder..."
Reaching the next landing, she took off down the corridor, speeding up to let Tonks play catch up if she wished. While Aurora may insist that she didn't care how she looked in front of others, she was vain to the core.