My Rottie has just turned one and has also just been de-sexed. I am always very careful to keep her lean especially as a puppy. I also have not over exercised her while a puppy. She has 2 30 minute walks a day. We do half an hour of training and playing a day. On the weekends her walks are 10 minutes longer and we do over an hour of training.
Before we had her de-sexed she weighed 33kg and was on 2 feeds a day. A bowl of dry and a bowl of wet plus training treats. The bowls are large stainless steel.
Now 2 months later she now weighs 32kg and is getting the same amount of food. Her walks are slightly longer.
My hubby loves big, bulky rotties but I think because she is a bitch she will be a petite girl.