there is an event called June drop. basically the tree aborts many smaller apples due to insufficient plant hormones. The larger ones will remain so you will still get a decent crop.

You may wish to improve the quality and size of your fruit further by thinning, ie. removing some of the crop.
RHS say: 'Cooking apples are thinned harder than dessert apples to obtain larger fruits; aim for one fruit every 15-23cm (6-9in). Dessert apples can be thinned less severely, with one or two fruits every 10-15cm (4-6in). Leave just one fruit per cluster. Thinning can be done using secateurs, long scisssors or with a firm tug between thumb and forefinger. Remove misshapened, blemished fruit or poorly positioned fruit and the ‘king’ fruit at the centre of the cluster which is sometimes abnormally shaped. Aim to leave the strongest and best shaped.'