I tend to think that Humans and Klingons are generally matched. There are some Klingons that are stronger than the average Human, but there are also some Humans who are stronger than average and can give a Klingon warrior the fight for his life. It may truly depend on how much strength conditioning and combat training either a Klingon or a Human has, IMO.
Some Klingons have been depicted as being more aggressive with a kind of berserker mentality when they go into battle, but I would argue that some Humans are like that too. It kind of reminds me of what Quark once said on DS9...
"Let me tell you something about Hew-mons, Nephew. They’re a wonderful, friendly people...as long as their bellies are full and their holosuites are working. But take away their creature comforts, deprive them of food, sleep, sonic showers, put their lives in jeopardy over an extended period of time, and those same friendly, intelligent, wonderful people… will become as nasty and as violent as the most bloodthirsty Klingon."
--Quark to Nog
Some Klingons have been depicted as being more aggressive with a kind of berserker mentality when they go into battle, but I would argue that some Humans are like that too. It kind of reminds me of what Quark once said on DS9...
"Let me tell you something about Hew-mons, Nephew. They’re a wonderful, friendly people...as long as their bellies are full and their holosuites are working. But take away their creature comforts, deprive them of food, sleep, sonic showers, put their lives in jeopardy over an extended period of time, and those same friendly, intelligent, wonderful people… will become as nasty and as violent as the most bloodthirsty Klingon."
--Quark to Nog