It's hard to predict conventions based on past years', because new conventions come, old ones don't; just because there may have been one large and three small ten years ago doesn't mean that last year wasn't 2 large and no small or that this year won't be one family reunion and nothing else. It changes year to year.
You can call the SD and they will tell you what's scheduled, though.
As for the weather and crowd size, well, it's Florida, it's school vacation and it's August. No matter which hotel you stay in, that means it's hot and the parks are crowded...but the Grotto pool is great! Not as crowded during the day as other resorts, with plenty of chairs and space for all!