My son asked me this.
If a moose fell on his back, such that his head, antlers and back were all flat on the ground, would the weight of his antlers be too great for him to right himself and get back up?
That's an interesting question...
In a natural condition a moose would be unable to lie flat on his back. Moose have a large hump above their shoulder blades that would prevent them from being in that position.
If by chance a moose were to fall over and land in a depression in the earth it may be possible that he would be unable to right himself. Another factor of course would be the size and shape of his antlers.
A bull with a very large set of antlers say of 70 inches or more may have difficulty getting up if one side of his antlers were pinned under something. I think this would have to be a very rare occurrence though. Again, because of the large hump on the shoulder of a moose, it would be extremely unlikely that a moose would end up on his or her back.
I have seen photographs where 2 bull moose had locked antlers and one of the moose was pinned more or less on his back and was prevented from getting up. In that case both moose perished.