My 5 month old weighs 11.1kg!she's exclusively breastfed and seems healthy.is she likely to be a massive child or is there hope she will slim down?

If just breastfed,: Not a problem, her ultimate weight is determined/ predicted by her parents bmi and lifestyle.

It's okay: Many breast fed babies are a bit chubby. Don't start solids too early. Milk intake will eventually get lower and the baby will slim down when she starts walking. Don't feed the baby juices as these are empty calories with little nutritive value.

Being exclusively: breast-fed the first year is actually a protective factor against obesity, even in the babies who appear to reach "maximum density" by 4-5 months. Once you start solids, it's all about genetics + environment. Just wean to an open-mouth cup as close to a year as possible. You may enjoy Satter's "How to Get Your Kid to Eat, But Not Too Much." available on Amazon.