D&D has an exponential power curve (as we all know), meaning that 10 2nd level flunkies will have no hope against an 8th level fighter (pick numbers to suit your taste).
The biggest problem is the power curve isn't the same shape across all classes. Well, that and the action economy heavily favors small parties against fighty types, since they don't get nearly as large AoEs as wizardy types do.
Your fighter8's hp is going to be, say, around 70 or so (assuming a con of 16 or so)?
If he's fighting ten sorc2s, he's going to be worried about them hitting him with magic missile.
... let's say he kills two a round (since they probably aren't clumped together, and he has two iteratives), and two of them manage to beat his init (which seems fair).
Fighter takes 2d4+2 damage (cumulative 2d4+2, average 10.5)
2 dudes die
Fighter takes 8d4+8 damage (cumulative 10d4+10, average 35)
2 dudes die
Fighter takes 6d4+6 damage (cumulative 16d4+16, average 56)
2 dudes die
Fighter takes 4d4+4 damage (cumulative 20d4+20, average 70)
2 dudes die (or possibly 1 - he's disabled at this point)
Fighter takes 2d4+2 from the scared shitless survivors, and is defeated.
Note that if he rolled poorly on his init, he'd take down significantly fewer of the newb mages. Obviously, increasing the newbs ganked/round helps as well - reach would work, combined with whirlwind or cleave, but whirlwind attack kind of sucks.