Unless you are a responsible breeder, there is no reason for you NOT to spay her.
You might think you have her secure but one of these days she will get out. It isn't a question of if but when.
You would be astounded to find out the amount of puppies 2 unaltered dogs can have in just 5 years if none of their offspring are altered. The number is 64,000 puppies!!
She will be healthier, happier and have a longer life. My GR is on the chunky side but she also has hypothyroid. If you are basing your love and desire for your dog on her weight, then you don't have the love and passion for your dog that the rest of us do here.

IMHO