I know it's a running joke in the community that Draco talks about Harry non-stop to his parents, and they're sick of hearing it... but hear me out...
Draco talks about Harry so often that Lucius and Narcissa subconsciously remember all these things. And as a result, when Harry and Draco become good friends/start dating, Draco's parents know more about Harry than Harry knows about himself.
Imagine this. Harry's going to Malfoy Manor for the first time and they just finish up dinner.
Narcissa: "Harry, dear, would you like some dessert? We all know how you get when you haven't had something sweet."
And Harry's just like..."what? I don't get like anything...do I? How do they know this?" And Draco is looking down at the table, praying to whatever god to make him vanish.