Since they're dried, they will expand a little in water.
They won't cause an illness (Freeze dried doesn't cause illness, just increases chances for constipation).
For any dried food, whether freeze dried or sun dried, it's best to soak it in some tank water until it's softer. This gives it a chance to expand before being eaten and gives you an chance to see if you're going to be feeding too much.
SBD and bloating that happens with dried food is mostly due to the food expanding while in the stomach. It's the same with pellets, but with the dried foods, it's easier to feed more than needed.