I've been trying out my son's MTB on the backyard. It's grass and sand but my son's 20" bike handles very well. Even tho I have to bend over a lot to reach the handlebars, I feel in total control when riding the kid's bike. I've even bunny hop it all the time and it's very easy. I could say it's even more fun that riding the road bike on pavement. So I decided to fix my own MTB and use it on the back yard with my son, but... when I try to ride my big MTB I feel like I'm gonna fall anytime. It's hard to steer, it drifts all the time (despite putting new tires on it), and trying to jump with it is a 90% failure rate...
Why it's so much difficult with the big one? It's because is a LOT heavier? Because it's a hard tail? They are both x-mart bikes so that's not it, I think.
Because they are XMart bikes, they are much heavier than virtually every bike available, therefore, harder to get airborne.
Also, you're going to need a really long post for that saddle in order to get full leg extension while sitting because the juncture of the seat tube and top tube being so much lower than level with the ground. If your legs don't completely extend at the bottom of each stroke, they don't rest, so you might as well walk. At least that way you'll be way less fatigued.