Really large snowflakes can only form when the edges of smaller snowflakes are still sticky and liquid. This happens when snowflakes pass through air that is above freezing temperature.
The watery edges of each snowflake act as a glue and allow more smaller crystals to join together to form large snowflakes. (The largest snowflakes can be thought of as lots of smaller ones stuck together.)
This means that the biggest snowflakes are a sign of mild air and, counter-intuitively, this means that snow is less likely to settle.