Sorry for the lack replies....
@Lily_pig until recently they were getting 1 of the sticks that you hang in the cage and i think they're are quite sugary but luckily because they are quite expensive I haven't given them for a while!

I have also reduced the amount of pellets that I give to almost nothing... I think they're trying to make me feel guilty because every time I open the back door they squeak really loudly at me!
@Betsy I've never heard of them storing fat before either! It also seems a bit too hot to be thinking about winter.
It has to be the sticks that are causing the problem. Like I mentioned, my guinea pig Molly gained alot of weight when I started to feed her treats excessively. I have seen those treat sticks for sale before and I know they are quite large. How often do you feed them? How large are the sticks you give them?
Overall, treat sticks are bad for guinea pig health. The treat sticks pet stores sell contain tons of grain and too much sugar for guinea pigs. Guinea pigs, being herbivores, are designed to eat only vegetables, mostly greens. Their digestive systems are designed to process fibrous greens we can not, such as grass and leaves. Sugar and grains, while liked many guinea pigs, are not part of their natural diet, and therefore they can cause weight issues if fed in excess.
Here is my advice: I wouldn't reduce the pellets; it is definitely the treat stick that is causing your problem. It is best to stick with feeding 1/4 of a cup of pellets each day and I wouldn't reduce the amount you give to less than this. I would also forage more for them and feed them plenty of what you pick (1-2 cups is what I would go for). Alternatively, you could feed this amount of fresh vegetables. Of course, it would be mean to eliminate treats completely. You could consider fruit to use as treats instead such as apple, grapes, etc. You shouldn't feed these excessively though, because they are very sugary. 1 small piece of fruit per guinea pig each day is recommended. You could also try giving them other treats like a raisin or sunflower seed per day for each guinea pig. I know it seems like this isn't alot, but because guinea pigs are not designed by nature to process sugary foods (they are herbivores by nature) and because they are so small in size, this amount per day is ideal for a healthy guinea pig.
Another idea I think you should try is making your own treat sticks for your piggies. If you do this, you can control what is in the treat sticks and add less sugars, seeds, and grains. You could make a stick made of healthier foods, such as dried carrots, hay, etc.