I'm 5ft 2, and have consistently loitered around size 8-10 since reaching adulthood. I've twice lost 2sts of "me" weight following the births of my DCs.
The double edged sword of my small build is that it doesn't take much imbalance to start showing on my face and waist. By 9st 2, it starts showing quite clearly on me. This means that I do regularly monitor my weight, and do reign it in quickly when I've exceeded my comfort zone. The plus side of this means that other than pregnancy, it's never been a big job, and just being extra careful for a couple of weeks keeps me where I want to be. I have never "dieted" as in following a plan to restrict my intake of food.
To sustain a slim build I:
Exercise- I enjoy it, and I need it to sleep
Stop when I'm comfortably full. I will leave two mouthfuls on my plate if I have finished what I need.
Eat moderate portions, see above
Only drink sugary/ alcoholic drinks occasionally
Think about the nutrional value of food rather than calories. I aim to have proteins, carbs and veg in most meals.
Nothing is banned, but low nutrition/ empty calorie foods are not routinely bought into the house.
Eat whole fat/ sugar versions of food. If I'm going to have something like coke, I'll have the proper red label version, enjoy it, but may not finish it.
Eat consistently. I'll eat a substantial breakfast of something like porridge that keeps my blood sugars stable. I'll then have school dinners, and a cooked meal in the evening. The calorie split is about 400, 600, 800. I may have a snack if I'm hungry, but not as a routine. (I know this from MFP from losing baby weight)
My tracker tells me that if I'm very sedentary, I'll use about 16-1700 calories, and up to 2200 when I've exercised, so on balance, my routine intake is in the right midrange compared to what I burn.
There is nothing punitive about my diet and exercise, I enjoy both. The habits I have keep me in balance fairly instinctively. If I've had a blowout lunch, I'll only need a snack in the evening and my balance will return.
I've had short phases of gaining surplus weight, and it's been connected with excessive sugary drinks and snacks, and overestimating the value of low impact exercise. One of my phases of gain was when I walked 6 miles per day at a slow pace, and I snacked more to keep my energy up. I do find that I enjoy treats more when they're occasional than when they've slipped into routines.
Some people manage to stay slim with no concious thought. I need some degree of mindfulness, but mainly have helpful habits, so I think the OP is incorrect about being permanently on a diet. (My colleagues definitely wouldn't say I'm in diet territory)