SPOILER
If you pay attention a little bit, when it showed scenes of the past they both looked the exact same. It's safe to call them twins, and this "curse" Meliodas has been trying to get rid of is most likely the cause of no longer growing.
Sooo... There's an actual chance that meliodas gets cured and finally turns into a proper adult...
(sorry, I just can't deal with his relationship with elizabeth as his kid form, it looks so... wrong and weird... )
I dont think his current form is kid like . I think u have a problem with the height difference between meliodas and the princess. Besides i think the princess also has the same curse but when she is reincarnated she has to grow up again and forget her previous past . Unlike meliodas.
So... Has anyone figured out why the hell there's a man who looks exactly as a grown up meliodas?
He refeered to Meliodas as "brother", but that doesn't seem the case.
Unless they are twins, but something happened to meliodas when he was a kid that made him stop growing. Gah, Idk...