Certified GKH Gardening Expert
Thinning may indeed be in order and this article will help with that: https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-thinning.htm
The problem could be that you are not thinning your peaches enough/at all. Some varieties need to be thinned because they over produce. You should thin peaches after June drop to 6" apart. Another cause could be lack of needed nutrients. A soil sample should be taken to your local extension office. Finally, peaches need a lot of water to size correctly.