Do being outside with no shoes on make your feet get bigger?

Well... technically, yes. This is because, a shoe that is too big for you gives your feet enough room to grow longer, wider, and healthier, faster. And usually, if your feet are given the room to grow, with no interference or any health problems, they'll grow bigger than they would with normal fitting, or tight fitting shoes. Also, with the extra room, your toes won't be forced closer to each other, and instead, they will grow to be long and thick, and spaced out evenly; the way are toes should be. AND, also, by giving your feet extra room, meaning, stopping any part of your foot to PUSH or STRAIN any part of the shoes, your soles and toes will reject wrinkles, and grow MUCH smoother, and they will become more senstive. And by them being sensetive, I mean that you easily feel something, or overly feel something. For example, if I were to barely tickle your foot (if it is as sensetive as a healthy foot should be) you'd die of laughter. Then... with the extra room in the shoe, your foot can completely develop. And, by this, I mean your sole's, and toes' skin thickening. Your skin will slightly thicken more than usual, giving you a slight amount of extra protection when you step on something. (This also protects you from warts, since it is harder to break through your sole's or toes' skin)

So, overall, YES, wearing shoes that give you extra room to let your feet grow freely, DO let your feet grow much faster. AND, usually, they'll grow bigger than they should. Plus, it is healthy for your feet, and for the appearnance of them.

But, if your aim is to have big feet, and you are willing to get dirty, just simply walk barefoot, A LOT, without washing your feet very much. The bacteria that sticks to your feet, if left there long enough, will cause your feet to grow faster, and MORE than they should. And, as long as you do not get a cut, or broken skin on your feet, (since many bacteria will cause warts through these) the bacteria actually will actually help your feet stay healthy, and will allow them to look more... appealing.

Also, just as an example, in a recent study, scientists estimated that:

If a boy/male were to walk around in the world for their whole life, BAREFOOT, but still washed his feet like they should be washed, the boy's/male's foot would be in between size 14 and size 16.

If a boy/male were to walk around in the world for their whole life, BAREFOOT, WITHOUT EVER WASHING his feet, (which is seemingly close to impossible to do unless your head's not screwed on right, and you want HOBBIT feet) his feet would grow to be in between Size 22 and Size 25.