I'll have a go
I have 2 ds's, both used BGs. My youngest is now 20 months and is still in the medium popper setting, he's slimming down but used to have chunky thighs and small middle. My eldest, when he potty trained when he'd just turned 2 was still in the medium setting, he's a bean pole. Without actually seeing the nappy on your ds in the flesh so-to-speak, it's difficult to know exactly what the problem is so I'm guessing it's one or more of the following:
I suspect it's a case of how the nappy is being put on. When you pull the nappy around your ds, the best way I have found (through some 2 years of trial and error
) is to pull the sides of the nappy around by pinching the top of the elastic on each thigh rather than by the fuzzy bit of the velcro tape. Does that make sense? I mean, pull the nappy closed by holding the sides by the elastic at the top of the thigh whilst keeping the fold (where the rise is poppered down) neatly tucked over. I think the leak sounds a bit like the leg hole on one side is being done up loose so that there's a gap.
I have found that if the insert isn't completely flat width-ways and ruched up, then wee can pool in the PUL and leak straight out the leg-hole. Could this be it?
My magic combo for inserts is the MF insert always poppered down to the minimum setting with the folded over bit toward the front in the wettest zone (boys eh?) and a hemp easypeasy insert underneath which I think is 6 or more layers of hemp jersey. This would my heavy wetting boys no more than 3 hours, sometimes more (max 4 hours) if they'd not been drinking much or if it's really hot, sometimes less (2.5 hours) if they'd just had a big drink. The insert I stuff in and pull back so it only just covers up the hosepipe, so not as high as the waist band.
I find the BG is best worn low slung, more like pants than a disposable nappy. It always comes up below the belly button on my boys, the velcro strip snugly fitting over the hips. Any higher and as the nappy fills with liquid, gravity would work the nappy down and the leg holes would work loose.
As a last resort, I'd look at wether or not you have a bit of detergent build up. It can make wee run right off the fleece and/or insert. The easiest way to determine if this is the problem is to check the drum when on the last rinse cycle and see if there's loads of bubbles. I've also heard of people running inserts under the tap to see how quick they absorb. Also check your detergent - if you're in a hard water area, if it contains soap, this can be problematic as it leaves a residue (like the soap scum you get on bath toys). I use bold 2 in 1, the infusions, about 25mls with a spoon of water-softening powder. I always put the detergent in the cold pre-wash and wash at 40 with higher water level and intensive stains with only the water-softening powder. The bold does tend to foam a bit, but I find on my machine, by the end of the cycle, the water is clear and the nappies smell scrummy.
Sorry for such a long answer, so many possibilities. Good luck, I hope you sort the problem.