Apples Getting Smaller Each Year.

Unless cared for on a regular basis, the fruit from apple trees will often get smaller as each year passes. This is often accompanied by a general paling of the foliage, but not always noticeable, as many apple tree varieties have pale green foliage anyway.

The cause of apples getting smaller fruits each year is usually as a result of lack of nutrient - Nitrogen in particular.

Things to do.

Clear the ground under your apple tree.
Feed it with Growmore or other general fertiliser. Or for a quick result in the growing season, you can apply a foliar feed as well. The foliar feed can be any liquid feed, but seaweed based liquid feeds are organic as well.
Make sure that you produce plenty of new fruiting stems by proper pruning each year - winter in particular.
Thin out your Apple fruit trusses to the best few apples per bunch.

It may be a year or so until the apples get back to their proper size, but at least it will all stop your apples from getting smaller each year.