Like John, I'm involved in Scouts where many times Be Prepared is interpreted to mean "Have a lot of stuff" rather than "Know a lot of stuff"
In that context, here is a good role model opportunity, that avoids any preaching and keeps to what Nick posted.
This approach is similar to what David & Tom posted, and apparently to what Mike C & GVP do.
Wait until you're into a nice long uphill slog , stroll up next to the heavyweighter, complement his/her pack in someway (plush hip belt cushion, construction, etc.)and ask to try it out for a bit. Swap packs. After two to three minutes (or so), repeat the complement of the heavyweight pack (plush hip belt cushion, construction, etc.), but make a quick non-judgmental passing comment - saying you personally prefer the comfort of being lightweight and signal it is time to swap packs back ... that many times opens up a lot of questions & dialogue ... and if not, well then so be it: no preaching, just enjoy your friends.