Induction Heater with different Mosfets amazing Improvement.

I replaced the STP40NF10 Mosfets with STP55NF06 Mosfets. I moved the physical location of the mosfets to the center of the LC capacitor bank. Mosfets have been moved 2.5" closer to the capacitors bank center , 50% closer to the LC coil. WOW what an amazing difference. Both loads 1/4" and 3/8" steel rods heat up much faster than before. The mosfets I removed were rated 100v, 40 amps, .o28 ohms, the new mosfets are rated 60v, 50 amps .015 ohms.

Why such an amazing unexpected improvement?

I put a .4 ohm resistor in the +15 volt line, between the DC power supply and the induction heater circuit. With the circuit ON but no load I get a voltage reading of 1.22v across the .4 ohm resistor. With the circuit loaded using a 1/4" steel rod meter reads 4.4v across the .4 ohm resistor. With a larger 3/8" load I get 5.18v across the .4 ohm resistor. Power supply is 15vdc. I think I should be able to calculate total current in the circuit from this but I am getting crazy number?

With the .4 ohm resistor removed and my amp meter in its place, idle current is 6.2 amps for the circuit. With the 1/4" steel rod load current is 18.6 amps. With the 3/8" steel rod load blows the 20 amp meter fuse. No load frequency is still 89KHz but load frequency has changed from 64KHz to 74 KHz.

I am trying to determine how many total amps the circuit pulls with the larger 3/8" load?

It is hard to know what all happened.
You moved the FETs and that probably made a difference.
0.015/0.028 is a difference.
The FETs are different in many ways (maybe)
Switching speed. Time to turn on and time to turn off.
Gate turn on voltage. If the voltage is different then the time when one FET is turning off and the other FET is turning on will be very different. (you might have both on for a short time)
There is a diode across the FET (inside) and I think some current is going through the diode. (the diodes are different)
FET loss is related to temperature. If one FET is hot and the other type is not hot then that is part of the difference.
The FET has "internal capacitors" if these are different they might tune the LC to a slightly different frequency.