I'm not sure if you are talking about a steel or soft bagger, but i have the grass gobbler (steel). I buy 45 gallon bags from lowes. They are a perfect fit to cover the back of the gobbler and dump. Tie the bag and throw it on the truck.
I dump into a large garbage can. 3 bags per can. When full I grab the can and kick the base with my foot to throw it to my shoulder. With leaves two cans,one in each hand to carry to the truck. It is easy on my back this way, not going over the side of the truck with the catcher. Not having to bend over to lift the barrel. Home depot has strong 32 gal. barrels for around $20.00
I have both a steel Gobbler, and a cloth bagger for my 36" w/b. For me, the best method to get clippings from mower to trailer is a tarp, two large dumps per tarp. I use 7X7, white plastic tarps from Landscape Supply (CA). They are just the right size for my height -- pick up all four corners and swing the tarp over my shoulder. The weight of the tarp is almost negligible (unlike a barrel), so dosn't add much weight to the load, and is very easy to carry back to the mower.
For my use, the 7X7 is the ideal size. A bigger one means the load is too far down the back of my legs, any smaller means that the corners don't come together very well. I'm not a strong person, so perhaps I am unable to carry as much as some of the rest of you.
first I don't mess with garbage bags. Waste of time and money. I lay a tarp in the bottom of the pickup bed before dumping any clippings into it. I unload the truck by pulling the front of the tarp over the top and rolling the load onto the ground. I will layer several tarps so I can handle the load by myself.
I also use tarps to dump on if I don't want to go back to the truck to dump. I use smaller tarps for this so no one tarp gets too heavy to throw into the truck. I tie the corners together in pairs before dragging it back to the truck. This keeps the load on the tarp and makes it easier to handle when throwing the thing into the truck.
I also use these need bags I get from Menard's (like home depot). they are made out of the same stuff as the plastic tarps, and are about 30 gallon size. they are collapsible, have a large oping and stand up by themselves once you get something in them. And the best part is that they only cost a couple of dollars each after a rebate. and they usually last at least a whole season. I think they are rubbermaid brand.