SEATTLE - You've noticed most of the time around here, the rain is generally light and the raindrops are small enough that you won't get too wet if you're caught outside. But then there are sometimes when the raindrops seem considerably larger and can leave you soaked in seconds.
Large raindrops mean that there are strong updrafts inside the clouds above your head. These upward blowing winds can hold raindrops inside the clouds for a longer period, allowing them to continue to grow in size until they finally become heavy enough for gravity to finally win the battle over the updraft, allowing the raindrop to fall to the ground.
That's why thunderstorms tend to have large raindrops, as they tend to also have strong updrafts. It's also why rain in the Midwest and East Coast can quickly leave you drenched.