my crabapple trees grow tall with a sparse canopy, but they don't seem to be sick or diseased.
The canopy of a tree usually reflects what is going on in the ground and thus how well the roots are able to absorb water and nutrients to send up the tree. I would check to see that there are not girdling roots and that the roots have been restricted or damaged somehow. Of course, then you can check all of the other environmental influences ie: supply of light, spacing, etc. You can always call your local extension office for help. Good luck.