T-Shirts & Basics
A t-shirt's fabric weight changes how "true to size" it reads more than almost any other detail on the product page. A lightweight, thin jersey drapes close to the body and can look smaller than its stated measurements; a heavyweight cotton in the identical cut holds its shape away from the body and reads roomier at the same size. Two shirts labelled the same size, in the same brand's own range, can look like different sizes side by side purely from fabric weight.
Fibre content also predicts how much a shirt will move after washing. 100% cotton, especially unpre-shrunk cotton, can shrink noticeably in length and width on its first few washes; a cotton-polyester blend holds its original measurements far more reliably over time. A shirt that fits perfectly out of the bag isn't guaranteed to fit the same after a month of normal washing if it's pure cotton.
A boxy, oversized, or cropped silhouette is a deliberate design choice on the pattern, not a sizing inconsistency to correct for — checking the product's own fit description ("regular," "relaxed," "oversized," "cropped") says more about how a basic will actually sit than comparing its size number to a fitted style from the same or a different brand.
T-Shirts & Basics at retailers

Old Navy
Graphic T-Shirt

Old Navy
Graphic T-Shirt

Old Navy
Graphic T-Shirt

Old Navy
Graphic T-Shirt

Old Navy
Graphic T-Shirt

Banana Republic Factory
Graphic T-Shirt

Banana Republic Factory
Graphic T-Shirt

Banana Republic Factory
Graphic T-Shirt

Uniqlo
Graphic T-Shirt

Uniqlo
Graphic T-Shirt

Champion
Classic Graphic T-Shirt, Script Logo - Big & Tall

Old Navy
Bratz™ Graphic T-Shirt for Girls

Old Navy
Short-Sleeve Graphic T-Shirt for Boys

Old Navy
EveryWear Crew-Neck Graphic T-Shirt

Old Navy
Crew-Neck Graphic T-Shirt

Old Navy
Sonic The Hedgehog™ Gender-Neutral Graphic T-Shirt for Kids

Old Navy
Pokémon™ Gender-Neutral Graphic T-Shirt for Kids

Old Navy
Disney/Pixar© Toy Story Gender-Neutral Graphic T-Shirt for Kids

Old Navy
Star Wars™ Gender-Neutral Graphic T-Shirt for Kids

Old Navy
Short-Sleeve Graphic T-Shirt for Boys