When a girl says that a guy "has a big heart," she is likely complimenting his kindness, compassion, and empathy. In this context, "big heart" is a metaphorical expression that refers to someone who is very caring, considerate, and emotionally generous towards others.
This compliment implies that the guy is genuinely concerned about the well-being of others, shows understanding and sympathy, and is willing to help and support people in need. Having a "big heart" is generally seen as a positive trait, as it indicates someone who is emotionally mature and capable of forming meaningful connections with others.
It's important to note that the phrase can have variations and may slightly differ in meaning depending on the context and the person saying it. But overall, when a girl says a guy "has a big heart," it's usually meant as a compliment that highlights his kind and empathetic nature.