Is one getting fatter then the other? Lol maybe they're different sex's

I had guinea pigs when I was young, I only fed them once a day from what I remember, but gave them heaps of grass for throughout the day- carrots are high n sugar so maybe not a good idea to give it to them all the time, more as a treat. Maybe try half a carrot each, pick a heap of grass and put that in their cage if they don't have access to grass all the time. They're like rabbits they just want to eat all the time!