If they are throwing fireball and firebolt every round, then that means they aren't casting other spells that are defensive in nature.
How much damage are they taking? How likely are they to avoid attacks when they aren't using their spells for purposes other than direct damage?
There are many factors to this. Are the foes able to move? Fireball is strong, but some monsters, especially at that CR can take one or two of them. What are these monsters doing in between Fireballs? If they are charging the wizard then what is the wizard doing in response?
If the wizard is continuing to fireball, then it is not mitigating damage against itself and will suffer from lower AC and hit point totals. If it turns spell use to protecting itself, then it is not outputting as much damage.
Meanwhile, the fighter is outputting its damage without any degradation of its defensive capabilities and it has the AC and hit points to accept incoming damage.
Your math is outside of any real (well game world real) situation. You need to define more clear parameters.