Meaning of "his heart had expanded"

Hi
Please guide me for the meaning of "... His heart had expanded unbelievably" here in this extract:
“When he ha d first walked through the wood with his uncle, they had stopped at that place and his uncle had pointed down to where, far across the field, a sagging unpainted house stood in a bare hard-packed yard. "Yonder it is, " he had said, "and someday it'll be yours-- these woods and that field and that fine house. " He remembered that his heart had expanded unbelievably. Suddenly he realized that the place was his. In the stress of having the boy return to him, he had never considered the property.”