Should someone hire an overweight person to do manual labour?

We've have a lawn maintenance company. We've hired 4 overweight people in the past. They've been fine until it starts getting warm out (over 25 Celcius). All the employees have to operate push mowers and line trimmers. Historically speaking, the overweight employees tend to start wheezing and being lazy when they have to physically exert themselves in the heat. This means that the slimmer, fitter employees have to do more work. I'm pretty slim and I can push mow in warm temperatures no problem throughout the day. It's cool outside now at this time of year. Should we hire an overweight person (like 35 lbs) if there's a good chance he'll collapse by summer. None of our overweight employees ever seemed to lose weight either.

There are no laws about that sort of thing in my province and we don't care about the race or sex of our employees either