No need to be worried. Milestones are not met at the same time by all kids. Wait for a couple of months and give her all support from your end. As u have already mentioned weight could definitely be an issue.
All the Best.

I agree with buggybun as I am facing similar delays in the milestones for my son. He is 9 months old and has just started crawling a week back. I did consult doctors and got the suggestion of having patience and supporting my baby as he was healthy in all aspects.

My daughter started walking only when she was 15 months old. ngShe did start walking when she turned 13 months old but after a couple of weeks she simply stopped walking. I was really tensed and had to go around meeting doctors to know the reason. Even doctors could not find anything. On consulting an Ayurvedic doctor he gave a oil to massage her body. I did the massage for a month and slowly she started walking.
I do not know whether this was the effect of oil or something else but it was a big relief that my daughter started walking. And by God's grace she is fine since then and keeps running around.

My daughter started walking once she was 11 months old but all kids have different ages for meeting the milestones. My nephew started walking only when he was 16 months old.

Nothing to be worried about but make sure that your child is healthy in all other aspect and then give a massage to strengthen the muscles and give them a chance to grow up fast.

There is nothing to worry here. My son starting sitting up at 9 months, walked around 13 months and could barely speak until 2. I have friends with children sitting at 5 months, walking around 1 year and speaking at almost same time but they eventually slow down somewhere. You will find all the kids around three years with almost similar achievements

As you are saying your child is healthy sometimes weight does become an obstacle in a baby's sitting and walking so you can wait for sometime.