Wow they are a good size. They are what, about 8 months old? That is what their weight should be on average for a full grown female. Perhaps they need to go on a bit of a diet? But if they are built like my Jesse much of it could just be 'mass'. It is definitely time to keep an eye on their weight.
As for heat, I think they will generally have their first heat at around 9 months old. Every 6 months average after that. Yuna is almost a year however, and she has not had hers yet, so that all varies too.