My Ford owners manual says set IBC to 6.0 to start. This would lock my brakes instantly, when using the manual control at 20mph. Even at 5.5, too much, would slide tires on concrete. I keep mine at 5.0, just right for my FW with 12" brakes. I can stop truck and FW, at lower speeds, using manual control only, with setting at 5.0.
I would suspect your trailer wiring may be light, or not making good connection at some point, or maybe just smaller brakes and hubs. I have my brakes adjusted to just make one revolution, with a light spin of wheel. It is about 4 clicks out, from not being able to turn wheel by hand.
Jerry