Desexing a dog doesnt necissarily mean they will become fat, however there is a deffinate pattern of desexed dogs putting on dramatic amounts of weight.
Without wanting to recite five years of vet school (lol) desexing a dog or a bitch, more so a bitch when it comes to weight gain alters their hormone levels, which in turn effects their metabolism.
Also, their is a pattern in desexed dogs leading more 'sedentary' lives post desexing (especially the males.
These two paired together = weight gain.
When desexing your dog you need to be aware of weight gain and adjust your diet as needed.
I have one girl here who put on about 6 kilos post desexing, and thats on a good diet! On the other hand, I have others who gained little.
Quite often vets will recommend desexing as a way to put weight on dogs, especially toy breeds.
As for the right age to desex, well my firm opinion on this is 6 months for females, ideally before the first season. Mainly because you near rule out chances of pyometra and other infections. Also, some vets will charge MORE after they have experienced heat cycles and the cost of a GA is largely dependant on the dogs weight, thus making it cheaper when they are 6 months compared to a year or more.
With boys, many people suggest 5 months is perfect as you can quite often 'snip them' before they learn to lift their leg. You will have tricked them, and many never learn post desexing and squat for life. Fantastic if you have boys that are prone to marking in your house.
My pet girls are all desexed at 6 months, my boys on the other hand stay entire untill I have an issue with them. In saying that though, I always ensure they are done before 4 to 5 years to help reduce the risk of prostate cancers etc
If you have any doubts, have a chat with your vet as peoples opinions differ greatly on this subject. The above though is based on what I was taught in vet school, and adheres to the Australian Veterinary Association guidelines.
Kyla