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

Uniqlo
Graphic T-Shirt

Uniqlo
Graphic T-Shirt
ASOS
ASOS DESIGN oversized Tennessee graphic T-shirt in blue
ASOS
ASOS DESIGN oversized graphic t-shirt in brown
ASOS
ASOS DESIGN oversized Tennessee graphic t-shirt in blue
ASOS
ASOS DESIGN oversized graphic t-shirt in brown
ASOS
ASOS DESIGN oversized Le Jardin floral graphic t-shirt in charcoal
ASOS
ASOS DESIGN oversized beautiful things graphic T-shirt in white
ASOS
ASOS DESIGN relaxed long sleeve ringer graphic tshirt in white
ASOS
ASOS DESIGN oversized relentless tiger graphic t-shirt in white
ASOS
ASOS DESIGN oversized relentless tiger graphic T-shirt in white
ASOS
ASOS DESIGN relaxed graphic t-shirt with Amis backprint
ASOS
ASOS DESIGN oversized beautiful things graphic t-shirt in white
ASOS
ASOS DESIGN oversized Le Jardin floral graphic t-shirt in charcoal
Lee Jeans
Mens 101 Rider Graphic T-Shirt

Old Navy
Buffalo Bills Oversized Graphic T-Shirt for Men

Old Navy
Chicago Bears Oversized Graphic T-Shirt for Men