My current liners from Ocado are 10L and do have some overhang over the top of the kitchen caddy, altought I find it the perfect amount for knotting and tying up before putting it outside. The bin doesn’t look like it’s as big as 10L, I too thought it was about 5L looking at it, but they must be deceptively capacious! I suspect that sizes of kitchen caddies vary across the country (including caddies bought independently for saving scraps for the compost bin) and the manufacturers of the bags have to accommodate this.
The Sainsburys ones are a little bigger because of the tie handles, but much flimsier.