Good things come in small packages!
I think Basil is so cute the way he is. You also have to look at his background to see how big he will be. Did you get to meet his mother or siblings when you adopted him? When we got our cat Sam as a 2 month old kitten we met her mother who was a petite 1 year old tux cat. We thought Sam would be small too but she obviously takes more after her father ending up at 14 lbs with some longhair mixed in too!
Isn't Noel a rather petite cat? And does Noah have something like Maine Coon in him? If so then expect Basil to be closer in size to Noel instead of Noah. No matter what you probably won't know for sure how big he will be for another year anyway!:)
And I love the photos of big brother Noah taking care of Basil they are so cute together!:D