Cotton shorts suit the kind of woman who wants to feel like herself on a hot day without thinking too hard about it. If you've got thicker thighs and you've made peace with that, good, because cotton doesn't lie, it sits where it sits. If you need something with a bit of structure for anything remotely formal, think twice, this isn't that.
Getting the fit right matters more than people assume with something so plain. I spent a good few summers buying my usual size and wondering why everything gaped at the waistband the second I sat down, turns out cotton with any amount of give needs to be bought closer to your actual measurements, not your usual size in a stretchier fabric. Check the rise before anything else. A low rise on cotton shorts will slide about all day because there's no elastane to hold it in place, and a high rise that's too generous will fold over itself and look like you've borrowed them. Watch the inner thigh seam too, that's where cotton shows its age fastest, and if the length sits right at the widest part of your leg you'll get a visible line that no amount of fiddling fixes. Go an inch longer than you think if you're worried about that.
What you pair them with matters just as much as the fit. Anything heavy on top, a thick knit, a structured jacket, fights the whole point of the shorts, which is to be light and a bit forgettable in a good way. Stiff leather shoes look wrong with them too, they want something soft underfoot, a canvas trainer or a flat sandal, not a boot. Where cotton shorts really earn a place in your summer is the ordinary stuff, a walk to get coffee, a train journey when the carriage has no air conditioning, sitting outside somewhere with a drink you didn't plan to have. They lose their nerve the second the weather turns, cotton holds damp and goes cold fast, so they're a June to August item for most of the country and not much use beyond that unless you're somewhere properly warm.
If you find a pair that doesn't need constant adjusting, that's the one worth keeping on rotation all summer.