I'm 5'9" and 11 stone or so, so hardly a big bloke, but I sweat like a dog in a Korean restaurant. It fairly pours off me. If I wear a cap, the peak gets so soaked that it drips. And it's not as if I'm really fast, either.
Same here with the treadie - no ventilation, environment already warm and sweaty.
I went out just ahead of the thunderstorms this evening. The air was so thick I could have chewed it. I got back home absolutely drenched.
But I love it! Standing in the conservatory, just back for a run, dripping away ... a great feeling. I don't think it's a problem but you do need to keep an eye on hydration.