I found mine only gaped when I was using a size too big for Nathan or if I left the waist too small.
Nathan used to have quite slim thighs and I found they weren't very good for him then but now he has chunky thighs they fit him great, if he was any skinnier I don't think they would fit him well at all and I'd probably have had to sell them on.
I've just bought my first AIO which I need to get in the wash and try as I've only every used the SIO and they've been great so far.
I thought the AIO did have bamboo inside, not just microfibre or is the bamboo in the stay dry layer? Nathan is a heavy wetter and worried that these won't hold up! Although luckily I have bought some mini boosters to add in...