I'm the slimmest I've ever been in my adult life right now, and very lean (visible muscle tone on my arms, legs, core), I have quite low body fat. I'm a size 8 and weigh 10 stone, bmi 22 - I'm not skinny/waif like, more like very lean. I've noticed the big change in the last 3 months since cutting out dairy, grains and sugar (eg 100% removed from my diet), it's stripped the final stone off me and really increased muscle. I don't snack, I eat 3 big meals a day, no processed food, no sugar, very limited carbs (eg a bit of fruit or potato, no more than one a day) and I eat only meat, fish, eggs, oil, nuts and veg. I feel very healthy right now. I'm not counting calories or limiting food in any way, except those food groups, and no snacking.
I run 35/40 km a week, do 3 hiit/circuits type outdoor classes, and yoga 3/4 times a week, but I did that anyway before I got strict with my diet and never got these results.
For me the difference between being a slightly squishy size 10 or a solid size 8 is clearly food. I was always dieting and never quite getting the results I wanted.