There's a reason why most weightlifters in the Olympic Games
don't look as polished as the bodybuilders you see in magazines. Bodybuilders don't merely have to build their muscles; they also have to keep fat from hiding them in order to achieve that chiseled look. However, strength is determined by the muscles themselves, not
whether there is fat as well; losing it is mostly a matter of showmanship for bodybuilders.
Also, keep in mind that in order to exert strength throughout the entire body, in other words to squat or deadlift massive weight, or to do the clean and jerk, the core musculature of the lower back and midsection have to be well developed, which increases the thickness of the midsection with muscle even before you add any fat.
Bodybuilders exercise muscles that are very rarely used for anything except to have "defined" physique, so they can actually be weaker, in practical terms than someone with an equal amount of muscle mass in more practical areas. A professional weightlifter's goal is lifting a lot of weight, so they work muscles which have utilitarian purposes.
Also, in order to be able to lift with the intensity seen by strongmen and powerlifters, one must consume absolutely massive quantities of food. Bodybuilders also do this, and if you look at them in the offseason, many of them appear quite chubby. It's only through starving and even dehydrating themselves prior to contests that they achieve their cut look. A bodybuilder is actually entering a contest at his weakest state, to such an extent it's not uncommon to see bodybuilders beginning to get faint or light-headed before going on stage. Champion bodybuilders enter contests at a bodyfat percentage of around 5%, which is physically impossible to maintain for anything more than a day or two. A weightlifter, not being judged by his appearance, but rather his performance, will not take these strength-sapping steps.
Fat itself can actually be helpful in a fight. Body fat, like muscle, helps absorb blows and can protect the body's internal organs as well as bones. Roman gladiators in particular are thought to have cultivated a layer of fat by eating barley so that their shallow, slashing blows could draw audience-pleasing blood without serious injury.
The physical ideal of having broad shoulders and a narrow waist comes from classical Greek artwork. In other cultures, such as the Japanese, overall strength was represented by a barrel torso, with samurai frequently depicted sporting large bellies. The idea is that a strong, powerful body needs to be firmly rooted in strong legs and hips/stomach. Which build is best for you depends on what you're trying to do. The classic V-shaped look is often called a "swimmer's build" for good reason and is generally the ideal for athletics where speed, grace and endurance are required. Athletes that focus on lifting or pushing, such as sumo wrestlers, linebackers and shot-putters, will invariably prefer the "stout" physique.