I don't think the general public even know that 10oz and 11oz mugs exist. They just order 'a mug' and that's it. It's only when they are given a choice that they realise there's a difference.
I think that is 100% correct. Customers don't know about size. It's just a mug to them.
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I use 11oz mugs, and I think the size is just right I currently buy 36 for about 92p a mug. If anyone has any cheaper suppliers than that, please enlighten me :-)