Grapefruit have been really small last 2 yrs. This tree has always produced great fruit.
Certified GKH Gardening Expert
The fruit may not be getting enough phosphorus and potassium in the mix. Alternatively, they could be getting too much nitrogen. Here is a link that will help for all general citrus: https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-fruit-thick-rind.htm