You know that babies grow fastest in the first year, cos clothes sizes last 3 months on average and then in the 2nd year slow down cos clothes sizes last 6 months? If a baby is going into 6-9 months clothes for example just after 1, then do you think they'd be in them for 3 months still or 6 months cos they'd grow slower?
All children grow at different rates. They have different start points. My daughter for example was in 3-6m clothes from being 6 weeks old and at 3 months now, she is moving into 6-9m clothes. She is big though. Your baby is just small, some children are.
Children have growth spurts at different times. You cannot know if her clothes at 12 months will still fit her when she is 15 months or 18 months. You'll just have to wait and see when her growth spurt is.