If an ender dragon with NoAI: 1b intersects a velocity-affecting block like water or lava, this velocity will not be cleared. As a result, it will grow boundlessly. Note that velocity will be reset when the ender dragon has AI re-enabled, so there shouldn't be any issue with the ender dragon attempting to move across thousands of chunks from built-up velocity.

Steps to reproduce

1. Run the following command to spawn an ender dragon:

/summon ender_dragon ~ ~ ~ {NoAI: 1b, Silent: 1b}

2. Place flowing water over the ender dragon
3. Run the following command to see the velocity of the ender dragon:

/data get entity @n[type=ender_dragon] Motion

4. Repeat above command and observe how the values change