Tartar sauce, for serving
Heat broiler, with rack 4 inches from heat. Line a rimmed baking sheet with aluminum foil. Set aside. Keeping each pound of crabmeat separate, turn out onto paper towels. Pick through to remove any shells or cartilage.
In a large bowl, stir together mayonnaise, parsley, lemon juice, egg, mustard, Old Bay,1/4teaspoon coarse salt and1/4teaspoon pepper.
Add crumbs and small-size crabmeat to mayonnaise mixture. Stir well to combine. Gently fold in larger-size crabmeat just until combined. Dividing evenly, form mixture into 8 cakes. (To make ahead: Arrange cakes in a parchment-lined container so they do not touch. Separate layers with additional parchment. Cover and refrigerate up to 1 day.)
Place cakes on prepared baking sheet. Drizzle with melted butter. Broil until golden brown and warmed throughout, 12 to 15 minutes (move to lower shelf if tops brown too quickly). Serve with tartar sauce and lemon wedges.