You don't say what sort of tree.....tiny or big. How old? If small......are there too many apples set, and root stock unable to enlarge the fruit? Thin it out a bit if too many. Apples on limiting rootstock are sensitive to anything, as they are on a knifedge in private gardens. Apple growers are better at it then ordinary gardeners. Watering and a bit of a feed too might help.

Increased watering (by you) is essential once the fruit has set if we're not having wet weather, so that the fruits can swell sufficiently.

You may need to thin them out, pick off the smallest or weakest looking to let the others grow bigger. If in a pot feed and watering are essential too.