Why are the back braces on my uke kit curved? The braces are too large for the top and back.
The back braces should be curved as you have noted. The back will then have a slight dome shape; it is not intended to be flat. A lot of ukes don't have a bridge plate; with the string tension that is present it's really not necessary. It's provided in this kit though, as long as it is glued between the braces and is under the bridge it will serve its purpose.
The braces are slightly oversized, the ones in your photos are the correct ones. Tapering the ends down to a very thin profile on the ends will help with the vibration of the top. They can also be scalloped or profiled to a smaller dimension if desired. Many uke builders trim the braces to "tune" the top, how much is carved away comes with experience.