With the younger/smaller Scouts, it's essential not only to fit them with a pack that actually matches their body size but that, when fully loaded, doesn't exceed your standards for percentage of body weight (perhaps 20%).
Sticking to this raises some issues, sure:
--increased need for ultra-lighting of personal gear
--larger Scouts might get gripy about allocation of gear to carry
--"one pack for life" is obviously not gonna work
I bring a set of scales to our preparation meetings and the Scouts take turns weighing individual items on the gram scale and larger items/full packs on the hanging scale. They compare and do math, based on their numbers from the bathroom floor scale and I make sure to have at least one large pack ready for parents to try on with 20% of their own body weight in it.
Then we talk about realities, being part of a Scout Team and the fact that if the group going can't carry all the gear without overloading the lighter Scouts, it can't go. This leads naturally back into how we can lighten our gear, such as the packs, the mess kits, the sleeping bags and the stuffed animals...