This seems to be turning into a 'it's much hotter where I live and I don't have an issue with it, so you'll be fine in Orlando in August' issue. And that's just not fair. Why???
Because it is really hugely hot and humid in central Florida in August. Actually most of Florida. Ah, this would be why there are so many hurricanes there then. Add to that normal heat/humidity, the fact that you are walking around, outside, all day long, on pavement. You are NOT walking out to your car in the morning, getting into that car, cranking the AC and then driving to the grocery store, where you get out and walk to the store, do your shopping and drive home again. Now, you break a sweat lugging in those groceries though. So, you pour a nice glass of lemonade and sit for a bit in your darkened cooled apt. But at WDW, you're outside for hours on end, usually starting early in the morning. You're ambling around, out in the sun, on nice pavement (so the heat just hits from all directions!!!) for hours. Then you may return to your resort for an afternoon swim. But the pool is tepid at best...but it's better than nothing. You spend an hour or two there, return to your room and shower and relax for a bit. You return to a park, and continue your trek from attraction to attraction...heat still sizzling underfoot...at least until 8 or 9pm. Even after that, the humidity is still high...you are still sweating at least a little bit. But it's better than being actually damp all over..right down to your undergarments during the day.
Really, it is different in the parks. The heat index in those parks, close to the pavement, is incredible. So, it is different...and people need to understand that. Heading to WDW, in June, July, August and Sept means expecting high heat/humidity and planning accordingly. Also....I have yet to see 'low' crowds during the summer. Yes, when you get towards the end of August, you see crowds die down. Why??? Families have kids in school now...they aren't heading to WDW. But try out the parks the first week free dining is offered during the summer months!!! OMG!! But, if you compare the crowds during Christmas vacation week or over Thanksgiving weekend to August crowds, then sure, I guess you can say they are 'lighter' in August.
Can you go to WDW in the heat of the summer and still have a good time??? Of course you can. But you need to be forewarned so that you can plan. There are things you need to do differently than you might in March, or November. Plain and simple.