My two cats, both boys, are the same age as your younger cat, booyhoo. One weighs about 3.4kg and the other 4.5kg. The lighter one is right at the bottom of the weight range for his age and sex, and the heavier one towards the top - they're brothers, father unknown, but they look very alike and completely unlike their mother, so almost certainly the same father. They have really different body types - bigger cat was a solid, muscular little thing, even as an 8wo kitten, while the smaller one is a slim little softy.
3.1kg seems like a fine, albeit light, weight for a girl the same age. You should be able to find some diagrams online of how cats look, when seen from above and from the side, if they are under or overweight, or just right. I did ask the vet about the smaller cat - she said he is just fine, and that the vast majority of the cats she sees are overweight, so people can be unaccustomed to seeing a cat of a healthy weight.
Oh, and he eats ANYTHING, to an alarming degree, unlike bigger brother, who is very fussy and has a tendency to throw up!