The bars are wicked wide. That doesn't work for me slaloming trees. The FS bikes do seem bigger to me. The kids are riding downhill pump style so weight and pedaling are generally not part of it.
They labor on climbs, burn gas, or push. The gear ratios are pretty good on a 2x10 and maybe even a 1x10 but I'd miss the big chain dump on the front when an unexpected climb pops up or the big ring if you get caught on a road somewhere. It does create a little clearance for log crossing.
I've done some bitching about what's become of one of my favorite trail networks because they
won't get off my yard groomed some of the best climbs for high speed downhills then throw the bikes in a shop van for work free cycling. There are some hardcore xc kids out there but the bulk market is for all mountain bikes. The money being spent for a quality xc bike is pretty insane. I've been plotting building up my '92 Jamis as a kickass xc bike but my head tube is not compatible with modern suspension forks.
What brand bikes are you seeing with the rectangular tubing? The bikes I'm seeing tend to be ovalized except for the chain stays which they claim are stiffer.