Well, it can be taught - I've done it with my horse a couple of times for fun - but I wouldn't suggest doing it if you're, as you put it yourself, heavy. It can really affect his balance.
About lying dow - when you teach this, you have to be sure your horse respects you enough not to start rolling, because otherwise you can get into serious injuries, if he starts rolling when you attemtp to mount him from ground. Also, the horse has to trust his human very much to be willing to lie down for him. As I did it - first, I taught him to lower his head ground level and not to pull it up again. Then, I started tapping the ground in front of his forelegs, until he started pawing ground and eventually lied down, however, if he tried to ignore the tapping and step forwards, I cued him to take a step back, then continued tapping the ground (rythmically) and, if necessary, gave a tap on his forelegs, under the knee, to encourage him to paw the ground. Keeping his head low and pawing the ground made him think of lying down, so eventually he thought of that by himself and now I just have to tap the ground a couple of times and he gets down.
But really think twice before trying to mount him from lying position.
I have come a long way, to surrender my shadow to the shadow of my horse.