“This year my apples are very small; is this a problem?”
Well possibly, but probably not.
Firstly, remember that some trees just produce small apples. Crab apples are generally plum sized or there about and are great for cider making, so you could be on to a winner.
If your spring was a good one, then perhaps too many apples got fertilised by the bees and bugs. You may want to hand thin the fruit next year to get larger sized apples. Take out a half of the apples (leave just 1-2 fruits per cluster) and the tree should then have enough energy to ensure the remainder grow big and healthy.
If your tree is producing small apples and there are also other signs of stress, such as curled leaves, blight, blackspot or bug infestation, this can account for small apples. The tree is just too busy trying to stay alive to be able to put effort into the apples.
However, chances are you have a crab or a tree that grew from a seedling and didn’t grow true to variety (which is the usual scenario for seed grown apples). If the tree is otherwise established and healthy, why not consider grafting other varieties to the root stock and enjoying pretty instant success in growing big juicy apples?
Or, better yet, graft on a mixture of apples that are good for cider making and have your own cider blend waiting for you to harvest each year.