I've run rotary cutters on hundred series tractors, early thousand series tractor, and our 3600 and currently my 4630, going back over 40 years now. It's one of the first jobs I was tasked with on this farm besides other work. Was my job to keep fields cut that were not in crops. I still do this at the current time.
On my 4630, there are conditions when cutting full depth, in the thickest, most dense vegetation, it would seem that this is all that tractor wants with a 6' cutter. Raise it up a little, that changes easily. I've run a lot of 5' mowers in the past and the same applies to the hundred and early thousand series tractor, but my old 850 tractor handles a 6' rotary cutter just fine. However, the conditions dictate how well, as well as how well it is maintained, adjusted and how low you cut. Try to take it all in one pass in thick, dense grass or weeds, up hill, all of these tractors will lug or bog down, it's purely up to the operator to adjust for the conditions. Heck, I mowed my lawn with the 6' cutter, set it down low onto the skids, then raised just a hair, did not turn and it cut my overgrown grass pretty darned good for a rotary, few skips and rough spots, but overall it took minutes when I did not have time, place looked kept at a glance and the lawn tractor cleaned it very easily the following week. I have photos of the results, who needs a finish mower LOL !!!
Ideal conditions around here are late May/early June, then cut again as needed or a bit longer until August. The height of the vegetation is less and the stalks are soft, water laden and supple. That makes for easy cutting. Cut mature, full height grasses, weeds etc. is much harder work, + you get all the chaff, dust and fuzz and spread weed seed everywhere.
3600 will handle a 6' rotary cutter just fine, but the operator must adjust to the conditions, the blades need to be new, or properly sharpened and the cutter must be adjusted correctly to the tractor. Cutting height must match the conditions, and if mismatched then performance will suffer.
There are too many variables, the tractor, the size mower, the conditions all play a role in sizing everything. My 4630 would handle an 8' rotary cutter, but try to cut to low, too fast, etc. it's going to be too much. With the 6' cutter I run with it, even in the heaviest, most dense vegetation, you can adjust speed, cutting height etc. and just walk through it while shredding it very effectively. I think a 7' cutter is the ideal size for this tractor in the conditions I have, but the 6' fits better in narrow paths and in the thick stuff, the tractor is not short on power very often, so again there are just a lot of variables to consider when sizing a tractor and cutter to the conditions you will work in.