Is it common for Chinese boyfriends to call their girlfriend “little sister” (小妹妹)?

I noticed my boyfriend addresses me as 小妹妹 or 小女孩 when talking about me to his family and occasionally to his friends. He does call me 女朋友 to his friends sometimes. But when he talks to me, he calls me baby (we speak English).

I am just curious, do Chinese men usually do this? Is it a term of affection? Coming from a different culture, I don’t get the calling your gf “little sister” so my biggest worry is that he is trying to hide or downplay the seriousness of the relationship by addressing me as “little sister”.

For context, I am 21 and he is 29. I need help from preferably a native speaker to understand the culture behind this.