What property makes some snow very powdery and fluffy, and some very easy to pack together and create a heavy ball?
Primarily the temperature.
If it’s very cold, the snowflakes are hard-edged crystals and don’t pack. As the temperature rises, these edges soften. Also, some of the snowflakes melt. The water acts as a binder.
And nationality.
Canadian snow produces superior balls.
As Reality Check said, moisture. Snowflakes that fall when the temperature is near or above freezing are melting and have a large moisture content. Snow on the ground when the temperature is above freezing also is melting.
Spray water on sand and see how the sand will pack and bind; the water causes that. I don’t know the chemical reason. It may be hydrogen bonding. It may just be a physical thing.
When it’s well below freezing, the snow is not melting, or hardly melting and there is no, or little, free moisture to bind the flakes together.
And don’t get me started again on how it can snow when the temperature is 40 degrees F, yet sometimes it rains (freezing rain) when the temperature is below freezing.