First of all, never try to make your Dog Fat coz with excess fat comes excess health issues....especially with Rotts which are very much prone to cardiac problems...A FIT Dog is the BEST Dog...and it's also dependant on the Bloodline/Genetics of the Dog.
2ndly, Females tend to be a bit slimmer as compared to the males of the same breed, also if you bitch is still under the growing stage you need not worry as she will gain the size n health she is supposed to when she reaches adulthood..
Now, as for Diet, if you are feeding your Rott the home cooked food then combine it with the following:
ROYAL CANIN's ROTTWEILER 31 - if your bitch is 2 to 18 months old and ROTTWEILER 26 if she is over 18 months...
Also, always make sure that CLEAN FRESH WATER is available to you bitch at all times and make sure that DEWORMING is done on time (consult you vet for the same)
Hope this helps...
Thanks & Regards