Not an expert, but let me have a few words.
What are you confusing about ? Just use basic power formula for the mosfet's power dissipation.
Power Dissipated by mosfet = Current^2 x Resistance
Resistance of the IRF540 is 0.077 Ohm (RDSOn).
For simplicity sake, lets assume the RDS ON is constant throughout the temperature, although this is NOT the case, check the other chart at "Normalized On-Resistance vs. Temperature" .
So at your 5 Watt which is at 1 Volt at 5 Amps :
Dissipated power by mosfet = Current ^ 2 x Resistance = 5^2 x 0.077 = 1.925 Watt
or merely just 2 Watt of heat.
Now at your 400 Watt which is at 80 Volt at 5 Amps :
Dissipated power by mosfet = Current ^ 2 x Resistance = 5^2 x 0.077 = 1.925 Watt
Same heat generated.
So for power dissipation at the mosfet, voltage does NOT matter, only the current passed through the mosfet and it's resistance. Not always the case though, read thru the end.
Another important thing to watch, see the note at your chart there, its stated the TC must be at 25C while the TJ is at 125C.
Another better example why you should watch for temperature, as it does matter, here the SOA of the mosfet at different temperatures, put few note in there too.
You will see at higher temperature, operating at higher voltage, the 900 Watt operating state (the tiny
red dot)which is at 90 Volt, when heated, it is moved from DC to 100 ms region.
To other experienced fellows, CMIIW.