As a BRTM on the smaller end of the body-size spectrum I can 90% of the stuff on my own...honestly, it has more to do with bulk/handling than it does weight. I can lift heavy stuff if I've got the little hand cut outs in the cardboard...but I've had stuff that's easily 15-20lbs lighter than the heaviest stuff, and I'm completely useless on my own, because I can't get a good hold on it.
As for carry outs, I'd probably call for a team lift on anything that was even remotely awkward/heavy, just because guests get super annoying when they see you handling "their" stuff.
As annoying as it is, I don't really blame them. Before I worked retail I think I had this mental image of everything being quintuple bubble-wrapped and handled with care until it was placed on the shelf. ...and I imagine most guests feel the same. Now that I've seen how stuff is really handled I'm kind of amazed anything ever makes it to the salesfloor in one piece. Most TMs understand there's a way you handle freight in the backroom/when the store is closed, and there's a way you handle freight when guests can see you. It's not that you're really being any more reckless when you're not being watched...you just take advantage of the fact that you know the limits of the product and how much 'abuse' it can endure.
So if I was working the front end and called for a team carry out, it wouldn't be because I was lazy/weak, but because I don't want a guest whining about how I'm handling their stuff.