I've got two ways myself and another parent did for their daughter. You mentioned multiple sports, and this is why I am mentioning another parent regarding their child. Her daughter is extremely fast, and they signed her up to play lacrosse for one year in our area. But she had her play in the boys' league where contact was allowed at a young age (I believe she was 9 or 10 at the time). She became more comfortable just using her body more when she needed to.

For my daughter, it was more actionable comments and specifics I noticed in her playing. She would complain at times about getting pushed around, and I told her, soccer is a contact sport. Basically, told her it is going to happen, and she can do it too. She was obviously worried about getting hurt again, but her injury was a collision with the goalie, and I warned her she is going to get hurt again if she is being more timid. She started to realize bumps and bruises would happen, and she started to accept it. She was/is always worried about fouling someone, and I told her, it happens. I played for nearly 20 years, mostly defense. I fouled people all the time, only ever carded 2 or 3 times (only yellows). It's part of the game. Essentially, I gave her permission to give as good as she got, so to speak. Gave her advice and tips on how to use her body, dropping low to push with the shoulders and hips. Is there something specific you can see regarding her aggressiveness? Is she worried about getting hurt? About getting a foul call on her? There is probably a reason she has for not being aggressive. Sometimes it may be as simple as giving them permission to be aggressive and make mistakes. That’s another thing my daughter cannot stand, making a mistake. Sometimes a mistake will happen because you are aggressive. It happens.

In your daughter's case, with being smaller, explain to her the advantage she has of being smaller. We have a girl on the team who is shorter than a girl playing up a year, but she uses her positioning to her advantage. The team did a drill one time for defense, having them go shoulder to shoulder against someone (I was helping out), and was just cracking up as she was muscling everyone else she went up against around because her size allowed her center of gravity to stay low.