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New tires are significantly stickier especially if your old tires were really old. So if you are trying to turn at low/no speed the you + power steering now has to overcome the increased stickiness of the tires.
This is normal.
I went from stock size 245 wide riken raptors to 295/35/18 continental extreme contact sports. So 8.5 inch wide front rims to 10.5 inch and stickier tires.
My steering is definitely harder and I can hear the pump working at times where I never heard it before.