Also there can be a fair bit extra in a 'proper' sized frame as opposed to one's built for pygmies..., eh Nick?
No, there isn't, that's a red herring, sorry Bruce! You're perhaps giving away half a pound between an XL and a medium, but that can come out in the wash in variations on other parts. At the end of the day the differences are in the frame, steerer and cables, that's about it! That doesn't add 2lbs or anything, despite what anyone who posts on a 'what does your bike weigh' thread wants to think! Some years back I weighed several Stumpjumpers at the shop, same model, a couple of mediums, a couple of large. The heaviest one was a medium...
The only thing is that someone riding an XL bike will weigh more than a shorter person of equivalent build, and thus perhaps need stronger components.
6lb short travel 29er alloy frame (eg spearfish) will take considerable work and money (XX1 or XTR 1x10 and LB wheels plus light finishing kit and lighter tyres) to get to a true 24lb with pedals.
See, you're falling into the trap! LB rims aren't really any lighter than Crests, you don't need to spend that lots of money to get a light wheelset (not that LB rims are hugely expensive obviously!). Sub 1400g would be doable on 29" Crests for <£400 I reckon. XX1 is light, but actually an XT/XTR 1x10 mix (with a T-Rex if you want) and a Next SL chainset would weigh less and cost less than a full XX1 or XTR groupset, if that was primary motivation. Obviously XX1 with a Next SL would be even lighter!