It’s the way that Lucy’s public persona and vibe is more reserved subtle smiles, and Lockwood’s is big charming grins, so while Lucy’s big grins are important and meaningful, for Lockwood it’s the opposite, his small subtle smiles hold even more meaning than his grins.
Like Lucy masks her emotions by toning them down and keeping them in (which makes sense given her home life) so when she’s really experiencing happiness and joy she full on grins because she just physically can’t contain it. But then Lockwood masks his emotions by performing, so when he holds back a smile it’s because it’s real and precious and he’s enjoying the moment just for himself and no one else and a small smile is just as, if not, more meaningful than a big grin from him.
And I think that’s so beautiful.