What causes stunted scabby apples?

So my MIL has an apple tree.  Each year the apples are small ,stunted and scabberous like the one pictured.
Any ideas what causes this?

The black spots are caused by apple scab. This although undesirable shouldn't be the primary the apples are small. There are quite a few reasons for small fruit. Do you have any more information on the tree and/or a picture of it.

How long have those apples had to develop this year?

My apple trees have apples about the same size but it is caused by later blooming this year.
One of our Arkansas Black apples bloomed about 1.5 months ago and we have one just now going into bloom.

As mentioned the black spots are apple scab, that can be controlled at the soil surface (remove all mulch and the top 3-5 inches of soil, that is where the scab overwinters).

one thing that can cause continuous small apples is soil compaction, another is low or non existent minerals (some good mineral rich sea salt and or kelp will help the mineral deficiency).

Redhawk

Perhaps thinning the fruit so more energy goes into fewer fruits?  Also prune the tree so that it's not a thicket, and direct its energy where you want it to go?

Some ideas.

sK

Or is it a "wild " apple that just has scabby fruit ? Or an apple grown out its usual range?

Generally valid and good advice, but I can't let the suggestion to remove all mulch andthe top 3 to 5 inches of soil to prevent scab, go unchallenged.  It is true that the Venturia fungus overwinters in the leaf litter from the apple trees.  (Leaves from other species do not harbour apple scab, and can be left as mulch.)  These fungi sporulate when spring rains splash the fungi.  That is, it is the surface fungi that are at play.  Actually, allowing the understory to grow up reduces the burden of contamination because it traps the spores in the undergrowth so they do not get up onto either fruit or leaves.  Removing the topsoil from the bases of the trees will have no beneficial effect, and considerable adverse effects.

More practical ways of managing scab are 1) spraying sulphur.  (I don't) 2) growing scab-resistant apples, (I do, some 50 odd varieties, mostly heritage varieties, but some recent cultivars bred for scab resistance.  I would highly recommend Williams Pride, out of the PRI breeding program, for those who want a bright red shiny apple with good flavour - totally scab-free.)
And I would second the comment from the French gentleman - if your tree is a wild seedling, it will make wild seedling apples, which are in the vast majority of cases, small and scabby - and sour.  But if that is what you have, the thing to do is top-work it to a desirable variety.  And best to choose a scab-resistant one while you are at it.