In Real Life, a person's physique, if not their physical size, is generally a dependable indicator of their physical strength. Muscle strength (force applied in Newtons) is proportional to the 'physiological cross-sectional area' (PCSA) or the total number of fascicles of the muscle.note All things being equal, more muscle translates to more strength.
In fiction, all bets are off. Muscles? Who needs 'em?
The Pintsized Powerhouse is able to physically outperform heavily-muscled guys ten times his size and is more than capable of sending them flying with a single punch, physics be damned. A thin, wiry character may have no difficulty lifting or punching way above their weight class. This is generally done to show just how badass they really are. Usually lampshaded by Super Strength, and often more dubiously by a Charles Atlas Superpower. Alternatively, The Big Guy may not be very strong at all, but usually his strength is simply dwarfed in comparison. The Big Guy being physically dominated is usually a Giant Mook or similarly unimportant character. Generally, when it comes to important Big Guys, Muscles Are Meaningful.
Weaker characters beating the stronger characters is often a demonstration of the fact that skill and other factors can trump strength in a fight, which is Truth in Television to a certain degree, but their methods have nothing to do with this trope.
There are several related tropes:
Mind over Matter. A psychic uses telekinesis to augment his own physical strength.
Inner Power. It's a common spiritual idea that inner strength equates physical strength. This is often used as a justification in universes where the spirit is the true source of a character's strength. This is famously known as qi (or ch'i) in Chinese gongfu (kung fu) philosophy.
Weak, but Skilled. The weaker character is more powerful because of superior technique. This can work as a believable justification until people start shattering mountains and punching out giant monsters.
Bishōnen + Beauty Equals Goodness. You can't have your Pretty Boy hero grow huge muscles without risking certain death at the hands of the Fan Girls ... and some of the Fanboys.
Crossing the Bishōnen Line: A character with multiple forms may get more beastly and muscular, only to suddenly take on a form where they're less heavily built, sometimes losing quite a lot of muscle in the process. Don't take these forms for granted, since they're often still strong enough to punch you across the room.
Super Strength. Characters may have big muscles in comic books to denote being super strong, but elsewhere this is often an exception. Sometimes even in comic books. In general, a "super strong" character with an average or only moderately buff physique will easily beat down even the most burly "normal".
Rule of Cool
Fake Muscles
No Guy Wants an Amazon. Muscles on women tend to be seen as unattractive and most creators care more about making their female characters attractive than their male ones. Hence women who can bench-press cars won't even be as cut as a typical athlete, let alone have the muscle mass that'd actually be necessary to perform that feat. Though even men with Super Strength are more likely to have a swimmer's build than a Hulk-like physiquenote , let alone the kind of large barrel-chested build real-life power lifters have (which hardly resembles what a body builder has).
Pretty Princess Powerhouse. The "pretty" part goes hand-in-hand with No Guy Wants an Amazon above. The "powerhouse" doesn't always refer to brute strength, but when it does, it often fits here.
Glass Cannon: A more realistic take on this trope. A character may have insane offensive skills due to their strength and maybe speed/agility/reflex, but their lack of muscle doesn't favor the character defensively once hit.
Supernatural Ancestry: The character might not have an ounce of muscle on them, but they can trace their family tree to otherworldly or even divine origins, thus explaining their absurd strength.
Note that this trope is specifically about instances in which the person with the seemingly weaker body possesses more actual strength than a heavily muscled opponent. This does not include characters who only win because of other characteristics that make them superior to their enemies, like speed or weapon proficiency. It's a common occurrence in Fighting Games that a speedy character is considered superior to the Mighty Glacier due to their speed and ability to perform Combos, but they only fit here if they are not only faster, but their attacks actually pack more of a punch as well.
Contrast Stout Strength, where the character has the muscle, he just has fat on top of it as well, and Muscles Are Meaningful, where the muscles DO make a difference. When it appears on comic book heroes, it is always a case of Heroic Build. Compare and contrast Clark Kent Outfit, when a character looks meek... until he takes his shirt off, and it's revealed that he has abs of steel.
See also: Bishōnen Line, Cute Bruiser, Little Miss Badass, Boobs of Steel, and Amazonian Beauty for specific character design examples. May overlap with The Gift, Hard Work Hardly Works, and Waif-Fu. See Monstrosity Equals Weakness for when this, and the other side of it, is the case across the board.
