I have a 5740C and I handle ALL sizes of round bales with mine. About two weeks ago I had a chance to buy 30, of the biggest round bales of pure coastal at $18.00 per bale. They were second cutting from this past year, but tight and solid, thus heavy. At that price I'm buying 20 more too. Anyway, they brought them to me on an 18 wheeler flat bed trailer, and they were stacked two high.
The FEL with my spike on it, stretched out almost as high as it would go, was able to spike that second teer just fine and raising it just a little, to its maximum, lifted those top bales up. I then backed up to get them off the trailer. At that point, with all that weight that high up, I immediately and slowly, let it down before I started to take it into the hay storage area for stacking. I have done it like this for several years now too and never have had a problem. These bales weighed in the range of 1200-1400 lbs.
I have no problems of any kind with those big bales, spiking them and moving them anywhere I want to take them on the place. I don't have weights on the back of my tractor or water in my tires either. Of course, I take care of my tractor and don't horse around with it foolishly either, especially when moving the weight of those big round bales. Just take care of the 5740C and it'll do just fine.