Yes your birds can get over weight on scratch if[/i] they are penned up and not allowed to work off the extra weight/ energy.. Yes there isn't much nutrition in the typical chicken scratch you would get from the feed mill/ farm store for egg production.
There are two types of Carbohydrates...
"Scratch" is a very generic name for a grain that can be tossed on the grown to make a bird "scratch" at the ground. it can be as simple as crack corn or a 15 grain mix.
For the most part the "scratch:" you will get at the feed mill (cracked corn, wheat, oats, & millet) will be a low protein...