Little raven, I have the exact same black bag as yours and had the same problem- the handles on mine were expanding to a point that the leather flaps holding the handle rings were starting to distort and strain the main body of the bag.
There arent any Gucci stores where I live, and the handles started to bother me so much that
I decided to do the crazy and took the handles out using mini screws! I tied ropes around handles to the desired width and soaked in water for few hours, then left them to dry in the sun for few days. After making sure the handles were completely dry, I screwed the handles back in. So far the handles dont show any further cracks nor mould, and have not expanded since. I think it's also important to keep them stored in dry, cool areas- which I admit I often forget to do!