A 1 1/4oz load in a 3" 20ga isn't comparable to a 12 ga load. The average 20ga 3" 1 1/4 is only doing 1187ft/sec and 12ga is doing 1300+ in the hunting loads. Not the same at all.
I do agree the 1oz and 7/8oz loads will take a pheasant down with no problems. The thing I would add is watch your shot choice. If you are going to hit a bird with less shot make it bigger shot. I never shot less than 6s at pheasant and prefer 5s or 4s at longer distance. I also shooting SKB doubles choke IC/IC over my pointing dogs. But have been known to choke at MOD/IMPMOD when shots get long. But if your really want fun shoot a 28ga.
Gselkhunter