Why, when snowing, do larger snow flakes indicate that the snow is about to stop?

What is making these flakes SO BIG? Is it the fact that they have a high water content and they're sticking together, or is it something else? Yes - high water content and stickiness play a big role. According to meteorologist Jeff Haby of theweatherprediction.net, large snowflakes are formed when temperatures in a layer of the atmosphere are just above freezing, causing the flakes to partially melt:

This produces a liquid film on the snowflake. This makes it much easier for snowflakes to stick together. Thus, it is liquid water that is the "glue" to producing large snowflakes and snow that is easy to make snowballs with.
Because large snowflakes are often associated with warming aloft, they're not a good sign for snow lovers. They often indicate snow is close to mixing with and changing to sleet or rain.

http://voices.washingtonpost.com/capitalweathergang/2008/01/the_cause_of_big_flakes.html

It seems that larger snowflakes are an indication of warmer air temperatures, which may or may not indicate the end of the current snowfall.

http://www.its.caltech.edu/~atomic/snowcrystals/faqs/faqs.htm
http://voices.washingtonpost.com/capitalweathergang/2008/01/the_cause_of_big_flakes.html