Why are labels so big when you want to post a parcel?

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Today I took quite a few parcels to be posted (I live in England) All of the ones that needed sending to the UK needed 2 stickers on, but a couple of them I had to rewrite the address smaller, so that these 2 stickers could go on, each one was about 3 inches square, so if you're parcel is 6 inches (a general envelope is only 6 inches long) then you have to either write the address so small it gets lost between the stickers or repackage the item in a bigger package, which can then make it more expensive to post. But to send packages abroad their are still 2 stickers needed, one which says AirMail and is about as big as that writing and one which is about 2 inches square. So why can't the postage stickers be smaller?