She doesn't look that small to me, she looks all grown up in the first picture
. From my understanding 18 inches (in height) is the smallest for females (going from akc standards). You also said her mom was on the smaller side so she could be too. And her weight (from just some general searching) looks be around the middle for female borders. She's also got a bit more growing to do too, maybe she just looks small compared to Levi? She could also being going through a lanky phase and her fur length is accentuating it