Lower wattage of IR bulbs permanently
When the bulbs are cooler i.e. running at "45 W" they have a lower resistance than they would have if run at 90 W. Hence use more current and more power than you would expect when run under these conditions. This is probably exceeding the specification of your smaller power supply. 

A power resistor will bring them back into the range for the smaller supply but this will need to dissipate a large amount of power. You are probably better off using the larger supply.