Think of your hairs as a bunch of strands of yarn. Tie all of those strands together into a mass of tangles and knots. It shrinks. Sometimes a lot. Sometimes not as much. But it always shrinks. You can't do anything to prevent it. Sometimes the tips of your hairs get caught by hairs higher up in the section. If enough hairs do it, this alone can cause a lock to shrink by half. Some people have even seem their locks fold over on themselves. It just happens.
Washing less often wouldn't really work because knots happen all the time. Washing helps them lock. But most knotting happens while you toss and turn in your sleep. You could wash less often and stay awake for days at a time. But that's not good for you.
Dreadlocks go through 3 major phases. The first few months they shrink. Then they seem to stay that length for a while. This is when your roots start to tighten up. This they just explode with growth. I had 1 lock that was 1 inch long when it matured. 6 years later it's about 18 inches long.
Shrinkage is something you'll have get used to. But it doesn't last forever.