When talking about gas, the word "puff" can imply several sized :
1. A cloud of smoke
2. A gust of air
3. The smoke coming out of a cigar/cigarette
4. A (human) breath
One of the definition being:
a small amount of smoke, air, or something that can rise into the air in a small cloud
So, on this case, Poison Spray is actually a quite small puff of noxious gas. The word "Spray" implies as much, in my opinion.
Personally, I see this spell as some sort of Pepper Spray...except it's poison.
Edit : Also, cloud can be used figuratively (a fluffy mass, not necessarily a giant mixed mass of gas/liquid/solid in the sky).
