Is my kitten overweight?

Recently I started to notice that my kitten is getting a little chubby. When our family first adopted her she was a tiny little eight week thing and she has grown a lot  since then. She's about eight months now so, obviously, she is going to be much bigger, but she also seems a little overweight. She doesn't actually look overweight, but when you pick her up she feels very heavy. We have four other cats (we are hardcore cat lovers) and most of them are smaller than her. We have three older cats who are our first cats. We got one cat and she had two litters of kittens and we kept one from each litter and then gave the rest away. And then those two we kept also had litters of kittens and so on and so forth. We ended up with five litters of kittens (over about 5 years) and we kept one from each litter, and made sure the rest of them got to good homes. Unfortunately two of our cats passed away, but we still have three left from those original litters. Those three cats didn't transition well to living inside and they're all related and a very tight knit family so, while they're affectionate, they aren't too interested in being with us humans. Because of this we have a situation set up for them where they can alternate from outside and the basement as they please and that actually works really well- they get lots of exercise and we give them lots of food and they're very healthy. The funny thing is our two inside cats, that we adopted as kittens from a shelter within the past couple of years, who get lots of attention from us and who we play with a lot (so that they get exercise) don't seem as healthy as our other ones. Our younger inside cat, Maggie, seems to be a little chubby...and our older inside cat, Scout, seems to be a to thin. Scout is a year and a half now and has been underweight her whole life. When we first adopted her from the shelter she was eight weeks old and only half a pound. Luckily we plumped her up a little bit and she seems healthy, but she is so tiny and light. She's a calico and from what I understand they're supposed to be more petite than other cats, but still. And our little kitten, who is also an inside cat, for the most part, is bigger than Scout and seems very heavy. So what can I do to improve the health of our two inside cats? We try to get them as much exercise as possible and they certainly aren't deprived when it comes to food...so what do we do? They do go outside sometimes, but not nearly as often or for as long as our other three cats that can come and go as they please. Should we set up something more like what we have with our other cats where they're outside more often? Thanks for any help.

What do they get to eat?  Your original residents, respective the new, mostly inside ones?

Although I suspect the residents are good chasers, and whatever they get at your place, they also eat quite a lot on their own.

Spontaneously, I wouldnt say a little plump is too much. Even is she is not a young kitten any more. Can you feel the ribs?  If you feel them clearly and can practically see them, and can easily count - too little, if you feel them barely - too  much, if you can feel them easily but not see easily - nice body shape.

For many moggie cats being a little plump is the normal. 

You have a comparison with many cats, but as you say, they are all relative to each other.

The two adopted are new bees, both with completely different strain...

Another reflection: 

I see you had solved the kitten question nicely.  But it would be swell if you neutered them more   vigorously...  

Good luck!