Q.Fruitless mulberry tree leaves sparse
Fruitless mulberry tree leaves are green but the leaves are half the size as last year. What causes this? My full grown fruitless mulberry tree use to have big leaves, but this year the leaves are over half the size. Leaves are green and have no spots but tree is very sparse. I live in CA, watering it same as I’ve done for past 3 years (more w/no rain or in extra hot weather).

Smaller leaves and less leaves can be a symptom of some sort of stress or root damage or inner wood disease.
The leaves are generally a late symptom and the damage has all ready occurred.
You can mulch around the tree with 2 to 3 inches of hardwood chips and put the tree on a regular watering schedule.
I would not try to fertilize the tree into health.
Wait and see.
Watch for signs of fungus on the tree and also around the base of the tree.
There is a disease called Armillaria Root Rot.
Here are a few links for you.
http://www.na.fs.fed.us/spfo/pubs/fidls/armillaria/armillaria.htm
https://www.gardeningknowhow.com/ornamental/trees/fruitless-mulberry/fruitless-mulberry-trees.htm